Same thing happened to me. New Apple watch 8 bought first week of Dec. 2022. Wore it every day but only for a couple of nights. Suddenly this red bump appears right under one of the sensors. Only ONE of the sensors. Looks like a perfectly round burn mark (seems to be filled). Does neither hurt nor itch. Called Apple support last week. 2 return calls so far with no new insights. Got an email so far on how to clean the watch….. So far they called it an „allergic reaction“. On what? The underside of the watch is glass or ceramic and the mark was exactly UNDER one of the sensors.
Watch was NOT tight at all.
Stopped wearing it over the weekend. Seems like the mark is slowly fading. Today turned all heart and other readings off and switched wrists. Curious whether the same will happen.
But I bought the thing for exactly those readings.
Next week will be another recall. I am very eager to know what they say…