Yesterday, I setup for the 4DP test. Kickr, headwind, iPhoneX, garmin HRM dual, configuration. Started my test and noticed my heart rate was showing 0bpm. Sufferfest app >devices saw a good HRM signal as per signal strength meter.
The headwind was connected via the HRM direct control via ANT+. It was blowing at the expected setting.
I attempted to delete sensor and re-add. no success.
Powered down my garmin fenix to ensure no competing BLE connection.
I started Zwift from an Apple TV. HRM was measuring correctly with either direct connection or with use of the companion app running on the iPhone.
Attempted to remedy with multiple iterations of powering off, resetting, etc. (This anomaly seems to only impact sufferfest’s view of the activity).The sufferfest app was deleted and reinstalled.
This configuration has been working through the "preparation for the 4DP’ plan earlier in the week.
Using an iPad mini running the same version of IOS (14.2), the garmin HRM successfully connected and displayed my heart rate. I was able to complete the 4DP test.
Today, I rode the Open60. Same symptoms while using the iPhone / Sufferfest. (HRM sensor appears connected, but 0BPM). With the iPad mini (5th gen) app able to connect and display my heartrate.
Any suggestions on how to T/S?
These dual Garmins are notorious for losing connection. I gave up on mine long time ago. Just get the single one with ANT+ only or switch over to Wahoo. It’s your sensor and not the app.
I have almost the same setup (Kickr bike), but also have same fenix watch. The only problem I had with it was when I didn’t connect my bike to sufferfest first. I’ve not had a problem since I figured that out. Not sure if that will help your situation, but it helped me. I also record all the data on my garmin 830 all at the same time - no problems.
Appreciate the responses. Though my experience has been mostly outdoors, I have not had any (previous) issue with my Garmin. That said, the outdoor experience is quite different with many less competing BLE connections possible. This appears to be a very strange anomaly. Shut down everything. Booted phone, removed and added HRM. Turned on Kickr. Loaded Sufferfest, connected and started app. Kickr connected, cadence connected, HRM connected and begun showing BPM. Started my workout. Paused, turned on headwind for HRM ANT+ control. Working. Light Therapy light turned on. EVERYTHING working! YES. Strange thing while riding on an indoor trainer. The smallest thing can create a distraction which is completely beyond what the actual situation warrants. It’s nice to now not have any unnecessary distraction! 
Now, to not F with anything and hopefully, it will be nothing but clear riding ahead…Thanks again, for the responses.