It’s probably just a wild guess, but somehow this reminds me a bit of the issue with the FP4 that randomly dimmed the screen when exposed to the sun. Which turned out to be some kind of overheating in the sense that one of the many internal thermal measurements of the phone exceeded a threshold that probably was just too low. In fact this could be reproduced by playing certain games for a few minutes and the phone didn’t feel overly warm/hot when the problem occurred.
Back then, somebody was able to find the measurement that caused the issue: Random Screen dimming (while brightness slider stays at 100%) after A12 update - #291 by LukeSkywalker
Ultimately this had to be fixed by an update. So if this case would turn out to be a similar case, I think the Fairphone support would have to be involved.
But I’m aware that response times of the support aren’t good enough, though.