FP3+ getting hot and reduces power during the last days

My FP3+ gets sometimes hot during the last days, though the room temperature is below 23°C. The hot area is in the lower part. LineageOS(+microG) gives me a short on screen warning that the power is reduced because the phone is too hot. I always have enegy saving activated so the battery lasts longer.

One time when the phone got hot I was in a building where the mobile data (4G) was available but quite weak. Sometimes even an audio stream broke because the connection was lost. I guess the modem was transmitting with full power.

Did persons with a sudden death experience similar problems?

Not as I remember, but have a look round:
https://forum.fairphone.com/search?q=fp3+sudden+death

Also have a look here, you may find some things that correspond to what you are seeing:

Let us know if we can merge your post there.