Hi. Forerunner 955 Solar with firmware 14.20.
I've got 2 x HRM-Pro devices. I'd previously registered both of these HRM straps to my watch (firmware 9.60) and could run with either one and the HRM Dynamics data would be captured in full for my run, every time regardless of which strap I ran with. This worked just fine on my watch last year and worked every time (and still works) on my Forerunner 945.
Now, I can pair one device but if I register both HRM devices, the watch won't record any HRM Dynamics data. I've just had pretty much the worst interaction with Garmin Customer support ever who say this is 'by design' and that you can't pair more than one HRM device to your watch. This information isn't published (they said it didn't need to be) but this seems like a MAJOR limitation of the 955 over the 945 which works fine capturing HRM Dynamics data when the watch is paired with multiple HRM devices