Running Heart Rate Splits

In this clip you can see FezzWheel, the world’s best runner of all time, running along. His heart rate stays constant at around 140 bpm.
However, the run splits on the left-hand side of his screen show a different story.
Is this a known bug or is there a simple fix? Thanks
(Posting on his behalf)

Link to Twitch Video Clip:

I have a potential explanation as it happens to me.

My HR mid run can report high, generally in the 2nd or 3rd mile. This will cause the split for that mile to report a high average HR settling down and the gradually increasing as the run progresses.

I’ve also experienced erratic HR simply if my watch is one notch looser than it normally is.

Based on the video I’m content that’s a genuine run.

I can tell this from other clues but I’ll not disclose these as i would give away tips on how you can cheat.