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That sounds like a strange problem to have at 20+ °C. Here is a thread where that happend but it was actually cold:
There is certainly a temp sensor in the battery. It could be faulty. Do you have a chance to test with another battery? Else contacting support (contactsupport) about this can’t hurt as this forum is not regularly monitored by Fairphone staff.
Edited to add: I realize that you don’t have warranty any more. But support might still be able to give you some insights into what the problem might be.
Also, I edited your topic title to convey some more relevant information. If that’s not OK for you, please just change it as you see fit.