I got this issue when connecting my Flux via BLTE. I ended up staying on an ANT+ connection to avoid this.
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Good tip. I’ve got an ant+ Dongle somewhere
Ant+ did the trick but now getting a different issue with my Ant+ dongle (it’s a cheap one so probably the dongle), where power changes will lag by about 5 secs, somewhat spoiled the Cure on Monday (There is a risk I am not cured). I’ve done the firmware back date on the Tacx and I’ve made sure my SYSTM app is up to date. Went to do 14 vise grips today and the issue came back (using bluetooth). Ended up doing to the workout as best I could on the road. I’ve raised the issue with both Wahoo and Garmin. Garmin seem reluctant to accept it’s an issue with the trainer.
So Ant+ only pings a signal every few seconds. If the ping does not align with the change in power indicated by the app, there tends to be a delay, i.e. resistance may only switch on/off a few seconds after the app tells you it should be switching on/off.
There is no fix for this, it is a quirk of ant+ signal frequency, just be prepared for a but of a lag. And, because the signal ping isn’t exactly aligned to the timing of intervals starting and ending, the lag can be variablez anything from 0 to around 5 seconds I’ve found.
The Bluetooth issue with Tacx trainers rolling off resistance while using SYSTM is a problem I have had too, and is a known issue, though the cause remains unknown. I spent a lot of time working with Wahoo trying to resolve it and we never could. Best is to ride using ant+ (your dongle isn’t the issue) and be prepared for a bit of a lag. 
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I’ve tried everything I could without success, including all the instructions by Wahoo Support.
If you find a solution for Bluetooth, please don’t forget to share with us.
Good luck.
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Ok thanks @DameLisa. I’ll go with the ant+ then. That said my laptop has now given up the ghost too, so I’ll be shopping for a new one tonight. The lag with ant+ is not too much of an issue, except on workouts like the cure. The lag meant what should have been 20 secs on 10 secs off, ended up being 15 on 15 off, which is quite a different, less hard workout. Oddly I never got the lag at the end of the interval.
Those short fast intervals are best done in level mode anyway, even if your bluetooth works
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When my Bluetooth works I’ve always found them fine in erg mode. It’s stuff like the rookie as well that you need ERG and the rapid changes.
Also had this problem yesterday. First time I’ve used SYSTM and was really disappointed. Impossible to do a decent workout as resistance and power output all over the place. Checked firmware and all up to date, calibrated trainer, tried again and still not working. Zwift works fine so can’t be a trainer issue. Did wonder if it was anything to do with WiFi as I have poor signal where I train, but doesn’t happen with any other app so don’t see why that would be the problem.
Use ANT+ and not Bluetooth and see if that sorts it for you
Looks like you did The Cure. Those intervals are very short and if the wireless signal is anything but perfect your trainer won’t always respond in time. Many trainers won’t cope at all with such short intervals. Try a different workout and see if that’s any better.
Using iPhone so not possible
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Tried again with a different one and it seemed better at first, then went wrong again. Short workouts like this work fine on Zwift
@Rob_ARCC Do you have any other apps open? With BT sometimes the trainer may be grabbed by another app. Make sure you are completely out of Zwift or the Wahoo Fitness App or anything else that connects. The less than optimal WiFi signal probably isn’t helping but you could also try downloading the video and see if that makes things better. I don’t know the ins and outs of the tech specs of the different platforms but I believe Zwift pushes more stuff to the local machine - at least for racing but not sure about workouts.
Yep, I concur with @JSampson the rolling off resistance on the Tacx trainers hasn’t been reported on phones by others to my knowledge so quite possibly something else is grabbing the signal, especially if it’s now happening in Zwift.
Fingers crossed closing all other apps (and switching bluetooth off and then back on) helps. Let us know how you get on.
Experience this with a Garmin Ant+ dongle and my TacX. Anything with sprint intervals, I switch over to level mode.
Anybody tried this recently ? Has it been fixed ?
Flux S → Bluetooth → iPad → Systm - Doesn’t do ERg mode properly
Flux S → ANT+ → Windows PC → System - Does ERG mode fine.
( I guess I could get my iPad out and try it, if nobody else has )
@ChrisBa There is an older thread that is similar to your issue that you may want to check out.
Generally when I read BT issues I think of what could your trainer be connecting to that you don’t want it to or that there is other interference that is causing issues with BT.
Definitely reach out to Wahoo Support as they have likely encountered the problem and can help diagnose.