This happened to me too for the first time last night! I‘ve been wearing my watch for almost 3 years now and it is the first time this happened.
I am quite experienced with dermatitis etc, but this is very clearly and visibly a burn mark. Plus I woke up during the night when it happened and felt that my watch was very hot. My skin turned leathery over the reddened part. It is burning and frankly, I find it annoying that there are commenters in this feed dismissing what people are writing about their injuries. There are 55 people atm who clicked the me too button, so this seems to be a prevalent and frequent issue.
I am hoping this was a one off incident with my watch, but I won‘t be wearing it anymore during the night, in case it does happen again. It took me a while to wake up and notice it and my mark is pretty bad now. It looks like it will leave a scar.