I’ll chime in, as much because of the Tailwind connection as the dropping HR issue. Maybe several things that piqued my interest, actually.
I’ve done HammerGel for somewhere around 2 decades, I think. No plan to change that, honestly. I’m 65, mountain biker since about mid-30’s and got back into road biking around 40, when I started a little mtb racing too.
So a few weeks ago, talking to a fellow rider in our area who does a longer race about every year (266 miles mtb, I think it is), said to me as we were talking about my struggles with declining power, “You should try Tailwind. That Hammer stuff is pretty old school.”
Can’t argue with him, but at my age, most things are “old school.” 
(I stayed with 26" wheels LONG after 29" was standard, rode my '98 and '99 model Klein Mantras into the ground, and when frame fails were finally FINAL, I relented and upgraded to a carbon fiber Trek Fuel EX 29. 7 years later, it’s still my joy on the trails.
Similarly moved to a 2020 Domane SLR6 summer of 2019 and am loving bigger, softer tires and comfort geometry. It fits my older body well, and I can still go fairly fast uphill, and can rip downhills better than ever before.)
SO, I wasn’t easily convinced about Tailwind. I researched some, still not convinced. Simple sugars led to stomach upset issues in my earliest mtb racing, and when HammerGel came along, it seemed to stop the problems. I’m not well disciplined at getting the food IN as soon as I should, and that is PROBABLY the biggest reason I sometimes have been running out of power during rides. (I have had some serious health issues last fall that lasted into early this year, but am finally seeing most all metrics getting back close to where they were. I’ll leave that out of discussion here.)
The friend’s perspective is that Tailwind works for him, and he said something like, “You just have to keep feeding it to avoid the bonk.”
I’m not likely to be successful with that, I’m afraid.
So I’ve STAYED with HammerGel AND (more importantly and more effectively) Hammer Perpetuem in my drink, along with their FIZZ electrolytes tablets in the drink bottles. I do NOT ride the distances @nicktick has been doing, but I probably do more annual miles, 7,000-8,000 miles per year last 3 years, including some mtb in those miles. My LONG rides, though, are century rides, and longest ever 111 miles. Elevation is significant, just did 100 miler 2 days ago with just under 8,000 feet of climbing.
I’ve found that I need to START OUT with 1 serving of Perpetuem and 1 FIZZ in each 23oz water bottle if the ride is going to be 3hrs or longer, maybe even 2.5hrs is a better target. For me, that’s around 45miles or more. There’s ALWAYS elevation here, so I probably average 15-16mph most rides, and down into 14’s occasionally if climbing is extreme. I pulled 15.1mph a couple days ago on that century. Solo, some wind, and a long stretch of low power after a tough 5.5mile climb that I bonked on the other side, and had a very gradual but very long ascent. I’d gotten into energy debt early in the ride and it caught up to me. I worked harder to get the Perpetuem in along with more HammerGel and I did eventually recover slightly, and the last miles were mostly downhill so I felt decent again and could make a little time up.
So, the think I want to avoid most is bonking, but equally important is that upset stomach that just about STOPS me from eating. That’s been a thing of the past since I started using HammerGel, UNLESS I just ignore the need to eat for so long that I may as well have not brought the gel along. I’ve done that more than a few times. But I really never do that anymore, I just let myself get “a little” in debt and then start having warnings and try to make up ground, and ease off the gas pedal if necessary. I DO think that the complex carbs and BCAA works best for me, and ESPECIALLY the Perpetuem on long rides. I LOVE doing long rides, seeing more countryside, climbing more mountains, etc. So I tend to overdo it. I’m slowly learning that I need rest days or at least easier and yes, SHORTER days.
Whatever it’s worth, I suspect that the introduction of Tailwind to your routine should at least be considered as a possible piece of your puzzle. What have you done before that? What part of the old methods (assuming they worked?) are missing now? Is Tailwind okay but you also need something else? I can’t help there, but I know what works for me… I just need to be more disciplined about keeping the right stuff going IN, not just carrying it along with me! I’m embarrassed at how many times I have HAULED THE WEIGHT of all that needed for long rides, but FAILED TO EAT IT! 
As for the HR dropping, I recently started what was to be a moderate 40 miler or so, but turned back at about 5 miles because the only 2 hills I’d encountered to that point showed my HR steadily DROP as I climbed these small hills. I should have gone up into 130’s or so, but went down into the 70’s bpm and then the 60’s on the second one, steeper than the first one. I wasn’t hammering but I was working. I rolled back down and then checked my pulse by hand, saw it close to what both the Edge and Fenix were showing, and decided to roll back home.
Took a look at data, couldn’t figure out any problems, and my own experience with failing batteries in these HR straps (Garmin HRM Dual) was that they tended to go HIGH on last 2 or 3 rides before any obvious fail or just dropping out.
It was a Friday and I actually scheduled an appointment with my doc the following week to get checked out. But talking with a buddy that evening and looking at data more, he saw that I had dropouts showing on that ride. I’d missed them but they were there.
Ended up canceling the Dr. appointment and checking things out with the HRM. The battery TESTED at 3volts, which SHOULD be no problem for good readings. But I put a new battery in (3.34volts, which is pretty typical for a new CR2032 coincell) and things seemed to be fine. Did a ride inside that night, and HR acted as it should with the ride workout. Also used an OLDER HRM Dual and it did fine, HR looked on target.
That was about 3 weeks ago. I’ve since noticed more dropouts, not all throughout rides or for very long stretches, but there, nonetheless, with that same HRM device. The OLD one has actually become my dependable one and seems to not show ANY dropouts, so I think the one that is less than a year old is actually defective, but it will show what appears to be accurate, realistic data for almost the entire ride, but close examination usually shows some dropouts of HR, so I can’t trust it, I’m afraid.
Whether you have a similar issue, I have no idea, but I was surprised to see this with mine. You might look close for dropouts (I think you maybe already said you did that) and you might check your pulse manually a couple times or more on a ride to see if it agrees with what’s showing on devices. Not easy to do very accurately, I’m afraid, if it means stopping and trying to count what is likely a moving target at that time. And it isn’t certain to catch a discrepancy that MIGHT only happen when the transmitter and receiving devices are “under load?” I don’t know how that process works, actually, and whether the devices draw more power from the battery when HR is faster?? Seems possible but I don’t know.
Best of luck figuring things out. I would NOT DISCOUNT the possibility that there is a problem with something that you need to get checked out. We ALL felt indestructible once upon a time…
And we ALL would LOVE TO FEEL THAT WAY AGAIN!
I have a friend who has always been very active, strong, fit, etc. And he began having a problem during a race and knew it wasn’t just a minor deal but had no idea what made him so powerless when stressed. Pretty sudden thing.
He went and got tested, had a 95% blockage of LAD (widowmaker) and had to can his vacation plan and go get open heart surgery.
Nobody who does the things we do thinks that could happen to them. Until.
So don’t fret, but don’t ignore if things are really different. I hope it will be a relief situation for you that things will go back to how they were, hopefully a diet thing, or the overtraining that has already been mentioned and makes a LOT of sense to us who don’t know the whole situation, but only you know how unusual things have been for you, or maybe NOT very unusual except for the Tailwind etc?
Trusting you’ll find some answers as you think it through.
