Oh no, I’m starting to get that too! I developed an Apple Watch rash on my left wrist since I started running more last year. It started in October, and even wearing sports bands, including those from Nike, makes it worse. On leather straps it doesn’t make it come back if it clears up Plus, it leaves behind crusty sweat stains on my watch that are tough to scrub off. I’ve been trying to wear it on my right wrist more and making the sports bands looser, and I’ve switched to medium/large on my sports bands if it had that longer piece .
It seems like the blood oxygen sensors might be the culprit, as they turn on when I’m doing yoga or meditation. That’s when sweat gets trapped as it cools off after weights and runs.