Every time I've had this happen to me or anyone else I knew, its because the wine version changed which can sometimes cause issues with the wine prefix.

I know you said no updates, but just in case you didn't factor in the version of proton/wine updating, that could be what makes it seem like its randomly happening.

The easiest fix is to blow away the wine prefix, but depending on the game that could also blow away any SAVE DATA or mods installed for that game. And depending on how you have lutrus setup, multiple games could be setup to use the same wine prefix if they're using the same version of wine.

If you end up thinking this might be the issue, and you have trouble finding a guide, just reply and I'll try to find a good one for you.