The new Xiaomi watch sounds like a winning combination- an 8 year old chip with a practically dead OS. At least they've come up with some original, cutting edge design.
I have the same issue with my Amazefit
It is most likely the material used to encase the sensor not agreeing with the skin on your wrist. I suspect it is a plastic of some sort. No one likes having plastic rubbed on their skins as they are actually quite abrasive.
Please go for ceramic backed smart watches. Ceramic is harder and smoother.