My F7x alerted a low HR but the watch was rested on a table for about half an hour....

runny said:

Never happened before

I dare say: you didn't observe it so far...it largely depends on what color your table has. I'm not joking, when we looked into the erratic behavior of OHR back when it started years ago, (you'll find many threads about that since the problem has been existing at least for 2 years for many, e.g. described here:  https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-7-series/408780/wrist-heart-rate-still-broken )

I found out  by accident that if I put it onto a yellow sheet of paper (a Postit at that time) it starts counting. Others observed a HR reading with the unit fixed to the bars of their cycle b/o the vibrations.

So this is not really new, I was able to reproduce it with at least 4 units with >/= 21.19 now. Probably this it was happens when developers try to enhance what can't be enhanced since e.g. a slight delay with OHR is immanent to the principle of measurement. Every effort to overcome that by approximation via software is just guessing and leads to other problems.

This one of the bugs we have to live with for at least 2 years and the reason why my "production unit" is still on 15.77 and I downgraded all units from friends and family to 17.28. All of a sudden all their OHR problems where gone as was the DEM bug among others...

Thank you for your reply, really surprising! 
what do you mean as 21.19 and 15.77 ? 

runny said:

what do you mean as 21.19 and 15.77

I refer to the software version of the Fenix 7 series. I was able to reproduce the problem from all recent versions beginning with 21.19 until now.  With versions 15.77 - 17.28 those problems are unknown hence some people prefer not to use the versions published in the last approx. 2 years but downgrade to versions with just some small bugs but not with real bloopers we got lately

If you want to know more about the firmware versions see this great change log here from our very helpfulk user Socorsu:

 https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-7-series/350860/garmin---fenix-7-epix-2-series---excel-changelog