There is a difference between:
I have no idea what temperature my phone runs at
and:
I have no idea how to check what temperature my phone runs at.
What I meant is: I pick my phone up X times a day, at no time have I felt that it was unusually warm, so at no time have I bothered to check its temperature - after 6 months of ownership.
In relatively idle operation, not charging, the temperature seems to be low-30s. I’ll check later today what the temperature is during charging (when it is ready to charge).
Should be easy enough to get a spare battery (from Purism) and fault isolate to see whether it makes a difference.
I wouldn’t be worried about temperatures in the 40s but 85C does seem out of spec!