why does my garmin hrm-pro plus drop it's signal on and off during a work out it is paired with forerunner 935

I may be having the same issue. Ever get yours figured out? These snapshots below are only from the past month. I need to go back to figure out when this started. I’m on a fenix 7.

Looks strange - the signal drop appears to happen very regularly every 10 minutes, so it is not just some random loss of signal due to an interference, dry contact, low battery, or similar. Don't you use some Connect IQ app on the watch? 

Try recording an activity with the strap standalone, in order to find whether the problem comes from the strap self, or rather from the watch. Instructions for doing so, may be found at: Top FAQs About the Activity Tracking Feature on a Compatible HRM | Garmin Customer Support

I have noticed a problem with drop outs on the HRM Pro Plus over the last few weeks both when cycling and when running.

For info: When cycling, my HRM Pro Plus is used in conjunction with my Garmin 1030+. I noticed when out on the road, my HRM status would be appearing and disappearing. 

When running, my HRM Pro Plus is used with my Garmin Instinct 2 Solar. I noticed yesterday after getting back from my run there were serious drop outs. The stuff that the watch records is fine.. but the stuff that the HRM records (stride, cadence, vertical oscillation and breathing) all show dropouts through the run.

ALSO: I have 2 HRM Pro Plus bands and I have tried alternating them to ensure that it is not a specific issue with one of them, I get the dropouts with both. Image posted below of my run data as it's easier to see the gaps across time than my cycling data (4 hour ride)...

mikem2208 said:

the stuff that the HRM records (stride, cadence, vertical oscillation and breathing) all show dropouts through the run.

In your case, I'd recommend changing the battery (even if the status still shows the battery OK).