HRM Pro dropouts and loss of dynamics data

I had my HRM Pro replaced by Garmin ~2 months ago, and for the first time today, during a structured workout on the 955, I've had a couple of dropouts that can be seen, and a total loss of running dynamics data from 30:15 into the run (as seen by the HRM Power recording value go from 0 ....as I was sat down recovering....to NULL). 

HR data is still there after this, but shows further drops in data after the efforts and going into the recovery sections between sets. 

955 and HRM Pro both on latest firmware versions.