Since I changed the camera module and put a new case (new model) on my FP2, my FP keep changing the mode.
Because of my job, I am almost always on “Do not disturb“ mode, but now my phone randomly changes mode…
It not only goes to vibrating but also with sound, the lowest volume possible.
As it always change to the same amount of volume, I assume it is not my fault, especially as it happens wether my phone is in my pocket or not…
Have you checked Settings > Sound & notification > Do not disturb access as well.
Maybe an app has requested access?
Although I am not really hopeful, that this is the cause of your troubles.
since you mention having changed the case, could it be that somehow the volume buttons are not set properly into their place and accidentaly press the button? because you can change the volume from vibrating only to low volume only by touching the volume button. So I would try to remove the case and carefully put it back in place?