I found this issue in discussions on earlier FP versions, but there is no solution (the problem vanished ..) Now, I have this problem with my FP6 (running /e/os 4.0). I keep missing calls, because my ringtone volume frequently goes to zero. I can turn the volume up again and then the phone rings reliably for some time but after a while, it goes to zero again. I think, the problem is related to connecting the phone to the fairbuds and/or Android Auto frequently, however, I never want the ringtone to be muted unless I am deactivating it.
Did you check with Murena for solutions, I think thats rather OS than device related and a 10 year old FP2 topic will most likely not help at all.