I have a similar issue with watch measurement (no extra heart rate monitor, only watch). During walks sometimes the heart rate suddenly starts showing a way too high. This looks like a watch issue to me, cause the only thing that fixes the problem is to take the watch off my wrist, wait for it to stop detecting the heart rate and then put the watch back on. Just changing how much the strap is tight doesn’t help at all. Example:
[image: 1771161988524-10da731d-b8a4-44e5-8c8c-f26b58f0f99f-image.png]