Audio bluetooth devices not playing sound on FP3+

• /e/ version: q
• Device model(s): FP3 (actually FP3+ but keeping the tag)

Summary

Bluetooth connection for audio devices does not work: audio goes to the device built-in speakers

The problem

Steps to reproduce

• register a bluetooth audio device (headphones...)
• pair them with the phone
• Confirm that under "Settings / Sound / Play media to" your BT device is listed as the audio target
• Play some audio and hear it come out of the device speaker

I've tested with audio from the built-in Media app, VLC and NewPipe. All give the same result, with 2 different bluetooth devices. On my other phone (lineageOS), all these combinations work fine.

What is the current behavior?

the sound is heard on the phone speaker

What is the expected correct behavior?

the sound should be heard on the BT device

Technical informations

Relevant logs (adb logcat)

Not sure what the relevant bits are, happy to test again if you can suggest what to grep for, or what sequence of actions to perform to get useful logs (there's just too much at the moment, I doubt it's all relevant)

Relevant screenshots

I don't know how to screenshot audio ;)

Solutions

Workaround

use a cable to connect the audio device, but it defeats the purpose of BT :)

Possible fixes

...