Strange you're being flagged since it's easy to find many other people on the graphene forums reporting the same (unresolved) problem.
Doesn't mean it's guaranteed to be limited to graphene in scope, but definitely seems to be a prevalent issue for some yet-to-be-determined reason.
Possibly related, does anybody know if graphene is vulnerable to Pegasus?
The easy explanation is that GrapheneOS users are obviously very interesting to Google and so they’ll slurp everything they possibly can, as often as they can.
I disable all Play packages in between installing the rare app on my (often elderly) users’ Graphene devices.
*Fans as in happy users, not fans as in blowing cool air over a red hot device
I had the opposite experience. I was pleasantly surprised by the battery life you can get when your phone isn't full of shitware apps and even when you do install shitware, the OS helps you confine it.