It’s probably the 3rd one because the temperature seems to be normal

and the reason ended up being either too old a phone (meaning it lacks the newer codecs and versions for Bluetooth)

I have a Galaxy A55 5G that is new enough to handle the Buds FE

or some bug in pairing leading to using the wrong codecs and/or protocol

Not sure since I’m not a codec expert but I checked and it’s set to this:

No

That’s why I use Windows 11.

No way that actually exists lol