I have the same issue!
I switched to Fairphone after being an iPhone user of 10 years and love it, I’m a complete convert but this is verging on being a show stopper. It is certainly putting my partner off from making the switch as well.
It is extremely annoying, you can be having a conversation with someone and the screen keeps going to sleep. Watching a video clip on Instagram and the screen goes to sleep. You get 25 seconds before the screen dims, then 5 seconds later it sleeps.
I have seen a post about being able to get around it by installing the open OS but I don’t want to go down that route for what surely is such a simple thing to resolve.
PLEASE fix this and allow me to sing the praises of the Fairphone to everyone I come into contact with, rather than saying ‘yeah, its good but until they fix the screen sleep issue I wouldn’t bother’.
I am keeping my fingers and toes crossed - I want to love this phone!!