If this is the same bug I had with my Run, then it is to do with pausing during a workout (auto or manual). See if you still have the problem if you disable auto pause and refrain from any manual pauses. It’s by no means a fix I know, but this is what support told me “For now, we still recommend keeping the auto-pause feature turned off, as it may occasionally cause irregularities in activity data”. I was seeing completely borked hr graphs any time I had any kind of pause in a workout, but it looked fine without.
I ended up returning my Run as I didn’t get much confidence it was going to be fixed any time soon, seems they are more concerned with fancy watch faces that fixing bugs