Just as a bit of anecdotal information for this topic. I went out for a walk yesterday evening and logged it using my S9PP. During the walk, I noticed that the HR being shown seemed to be a bit ‘off’ compared to what I perceived to be correct. The watch was quote close to my wrist bone, and so I shifted it back a little. The result was much improved data, which I guess just shows how critical this placement is.
By the way, before anyone queries the ‘perception’ of what was correct, I was also wearing a Garmin 955 for comparison - I just didn’t want to mention it