OK. My initial reply was based on a quick check using the internal speaker.
There may be a bug in negotiating volume over BT with your specific device. I have made the following check:
- Used “Signal Generator” app to provide a constant level of sound.
- Connected a BT headset.
- Checked the sound levels again.
When increasing or decreasing the volume using the physical buttons of the phone, I observe a 1s approx. lag at the headphones (doubtless due to the BT connection).
Number of steps is the same, 15, and as far as my ear can judge, they are acceptably distributed in acoustic terms.
Does the problem always occur between the same two levels, like going from 12 to 13?
Yes I agree, but isn’t that usual? I’ve never given it much thought to be honest, but surely that’s necessarily the case.
There’s a setting in Developer options called “Disable absolute volume” which may or may not help in your case (I rather suspect that AV is not being applied in your specific case anyway).
I think that more data from a variety of BT devices would be required to check all this out, but if you feel that you have enough to support a feature tweak request then the best would be to get in touch with Fairphone official support, since they don’t read this community forum on a regular basis. Click: contactsupport for more info.