Wildly inaccurate heart rates after ending workouts with HR monitor

5829726 said:

It might have something to do with the watch changing over from an external HR monitor to wrist HR at the end of the workouts.

This exactly what's happening here. Watch HR sensor starts measuring from 70 when it detects pulse. Same drop would be visible e.g. if you were using your wrist HR during activity and you took of your watch and put it on again. This is not documented anywhere BTW.

Same is happening here. I was out running, and in the middle of the run my HR "reset" and dropped down to 60-70 while running. New Forerunner 965, used for 2-3 weeks

I think you're right about it. I had it happen again today and I restarted the watch, hoping to get it working right. The HR app started off with ~80bpm then corrects itself (correctly) to ~120bpm after ~30s.
That...raises some serious questions about the accuracy of their non-active HR readings. The watch seems to be doing a fair bit of extrapolating...

I have had some very strange HR results too - big spikes way over my MaxHR at the end of an activity, and very low readings if the connection to the external HRM drops for some reason.

5829726 said:

That...raises some serious questions about the accuracy of their non-active HR readings. The watch seems to be doing a fair bit of extrapolating...

To be fair, I think this could be considered as normal, since they need to start from some sort of a baseline after reset or after you put your watch on your wrist. Optical sensor works fairly well when you wear the watch - i.e. it shows realistic numbers, even if it's not as precise as a chest HRM.

Obviously I'm just making assumptions based on anecdotal experience here and only Garmin knows how their algorithm works.

I don't think its an issue with the strap as low drops are when it drops back to wrist HR

Watch will  get the correct HR from the strap, drop the connection and show -- then wrist HR kicks in (but reporting too low) then the strap reconnects again and the correct HR is shown again.

I have also seen unfeasable highs at the end an activity after the external HRM correctly disconnects once the activity is ended and wrist HR kicks in but WAY over my MaxHR. Then it settles back to a typical post-activity rate after it locks to something more sensible 

My watch (965) consistently drops the HR to 70 after about 11 mins of running and stays there. Already had my watch replaced once by garmin and the new watch is having the same issue. Waiting for them to find the root cause, taking a while..