This used to happen with my Fenix 6 but I never posted about it. It happened again today on the new 955 so here it goes:
If you start a run without your HRM pro connected. For me, it would be that I had it turned off on the watch. You then connect it at any point in the activity, for me it was near the beginning, I get double recovery time. I thought it was a bit odd but you can see from the snapshot from Runalyze that the activity file actually shows the correct recovery time of 25hr, but my watch posted 50. I am now at 45. See picture.
This seems to be device wide, but it would be great to get a patch. It is also possible that had I not downloaded the HR data that it would not have done it but I did want the data and shouldn't have to chose.

I have to turn it off due to another issue I have where after a long sweaty run sometimes the chest strap will stay connected to the watch and give crazy 220+ HR readings. That seems to be an issue with the device but it would be nice if once an activity was done the watch would stop trying to read the HR from it or disconnect completely.