[novok]

My theory is since the screen is held to the watch with glue [0], heat exposure will cause the glue to unset slightly and break the waterproof seal. So over time your watch may lose it's seal from exposure to being in a hot day.

The malfunction happened to me after owning the watch for 14 months, and I swam with it in the ocean on month 4 with no problems. It was also fine on the first day, the issue started happening the day after. My theory is a very small amount of water got in, but it was enough to damage the oled screen inside.

The solution would be not using glue, but using screws or a bayonet on the back, but then it won't look like a seamless object. Maybe in the new non-jony era of apple they will make their watches properly waterproof? I doubt it although.

[0] https://www.ifixit.com/Guide/Apple+Watch+Series+3+Display+Re...