In the latest version of the software (20.19), it sometimes displays (and records) zero heart rate if you pause an activity, put the device to sleep, and resume the activity after some minutes. The only solution to this is to completely power off the device and start it again. The activity is not lost, but you have to constantly
A malfunction of the HR monitor is ruled out, as the heart rate is properly reported to my fenix 6 Pro at the same time that my Edge 840 shows zero beats per minute. I use the same HRM Pro+ for both of them.
I write it here because I didn't find a place to properly report bugs, with FIT files and logs.