When I spin a Sufferfest program on my 4iii, my garmin interprets the cadence as half of what I actually rode. For instance, if my average Suf cadence is 90rpm, when the ride data is uploaded from the app up onto garmin connect it summarizes the ride as 45rpm.
Oh, and it tells me that my training load is nothing.
What gives?
Firstly, welcome to the forum 
For the training load, check out this article by DC Rainmaker:
In short, you need to own a Garmin device that does training load calculation and it will show 0 until you sync that device AFTER the workout has transferred to Garmin Connect. That’s basically because Garmin do those calculations on their supported devices, not on Garmin Connect itself.
I’m not sure on the cadence issue, I’ve only ever seen something similar with my Stryd running power meter as in running there are essentially two definitions of cadence. For that, in the Stryd settings you can switch between the two options. But I can’t see that being the case with a bike power meter :-/ Plus I’m guessing you’re seeing the right cadence in Suf and it’s just after that activity goes to Garmin it’s showing half?!
Thank you aerobrain!
Yes, that is correct. The cadence is correct in Suf but wrong when translated to Garmin.
Regarding the load, I have a fenix 5s Plus. Thank you for the article reference. I’ve updated the software on my watch, and am eager to see if that changes how the load is registered next time I ride.
Hey @crashmartin, also Welcome to the Forum and to Sufferlandria. You can never leave. I had a single sided 4iiii Precision on my bike a while back and never had that issue. My facetious thought is that you must have been doing the single leg drills 
Seriously though, have you checked to be sure you are using the latest firmware for the 4iiii and is the PM being picked up by the Garmin for both Power and Cadence and does it register the correct cadence while you are riding? If SUF is showing the right cadence but Garmin isn’t, I suspect it has something to do with how the Garmin is recording it, so, be sure you are using the most recent firmware for the Garmin as well as the 4iiii.
Last bit of useless advice…if you still can’t get it sorted, you may want to reach out to 4iiii support directly. I’ve found them to be super responsive and very helpful whenever I needed them.
Edit: I see you updated the watch firmware so that may also solve the cadence issue. I am curious how this turns out. Please come back and let us know