I’m in the same exact case ! Working home and noticing the issue as the sun hits the watch face in always on mode (series 5 gps, WOS 6.2). Done some research, it only seems to malfunction when transitioning from shadow to sunlight as the screen is asleep, doesn’t do it with screen on. Temperature doesn’t seem to cause the issue, nor water damage as I never got it under water. It could simply be a general defect Apple doesn’t want many to know about. Shame... turning the screen on then putting it to sleep as in direct sunlight seems to temporarily fix the problem btw, but can’t be sure. If the problem was only software related, then it would have been addressed already, not promising...
Have a nice day !