I have not received any update in the past days since my last post. The FP support people are supposed to work on the topic.
Just now I received an update in my ticket: They ask me to
Charge to 100%.
Reboot into Safe mode.
Go to Settings → Battery → Battery usage → Take a screenshot.
Use the Fairphone in Safe mode until the battery goes below 15%.
Go to Settings → Battery → Battery usage → Take a screenshot.
Send us the results.
But the last time I tried to run the phone in safe mode some apps did not work correctly anymore afterwards. And as it is my daily driver phone I can hardly do that for a large part of the day.