I’m having exactly the same behavior, have you ever figured this out?
For me it started today, after about 5 months of no problems with the case. It seems last night there wasa power outage, this morning I found the RPI off. Turned it on with the button and noticed the fan was spinned up but never stopped even if temperature was around 30ºC and the config is the default.
Tried reinstalling the tools, apt-upgrade and a few more things, nothing worked until, after reading this, I pulled the plug.
I’m trying to figure out what the hell might be causing this nonsense behavior… but there’s so little information about how to troubleshoot I’m leaning to move to the one proposed in this thread rather than figuring out the pyhon script