I’m informing you on behalf of my wife who I convinced to buy a Fairphone (4) a few years ago and who’s mostly happy with it today 
She observed the other day that her mobile got warm / hot on the backside. It was difficult to pin-point the offending app. But after she observed that the display (glas) peeled off from the display (module) she turned on developer mode and limited the number of background jobs to contain the issue.
I couldn’t observe similar behaviour with my Fairphone (5).
I’m reporting in hope this will inform the next generation of Fairphones. I imagine that there might be a temperature sensor included near the display that instructs the OS to kill processes once a treshold is passed.
I am aware that it is possible to inspect Android using ADB but not everyone is so technical inclined with software.