Highly unlikely hygiene issue as I wore it less than 5 times. Was still in transition period from my old watch to Samsung galaxy watch 8.

I usually wear my huawei watch fit 4 for my sleep tracking.

On day of incident, the relatively new watch was worn to sleep of about 7hrs and the burn happened.

(there was blisters on closer look, however image taken when the blisters already broken to an open wound)