This problem has also been discussed in No ringtone with bluetooth headset connected . It does not only affect “ring tone sound” but also “alarm sound”, while connected to bluetooth. So if you are listening to music, you might miss an alarm ringing.
In Problems with Bluetooth (Murena Fairphone 6) - #39 by Manoj you can read: “The developers are putting in some fixes in v3.3 . They will also be checking some of the other bugs that are linked above.” Let’s hope, this bug will be fixed, too.