It might as well be another Android 9 issue: https://www.bestusefultips.com/fix-bluetooth-issues-after-android-9-pie-update/
The post is not the most helpful, but it does point out that there are more devices out there having this issue than only FairPhone.
Another thread which describes my same problem: https://forum.xda-developers.com/showthread.php?t=2516763 - from six years ago…