Volume control via mpc / ncmpc sometimes is ignored

I installed mopidy as a replacement for mpd. Installed versions are:
mopidy/stable,now 2.2.2-1 all
mopidy-alsamixer/stable 1.0.3-3 all

I use volume control via mpc from remote machines. I always send commands “volume -5” and “volume +5”. But sometimes, these commands seem to be ignored.
Even when I log in to the mopidy machine and try to control the volume via local ncmpc, volume control commands sometimes are ignored (I have alsamixer open in parallel to see if there are changes).
Also, what from my point of view really should’nt happen, is a misalignment between alsamixer’s volume and the volume reported by ncmpc. For example,
alsamixer says 76%, ncmpc says 84%.
I push the right cursor key for “louder” in ncmpc -> nothing happens.
I push it again -> nothing happens.
I push it a third time: the sound card as reported by alsamixer is spontaneously set to 80%, volume in ncmpc is reported to be 87%.
Iset the alsamixer min_/max_ volume values to 0 and 100, scale to linear, so there shouldn’t be problems.

To conclude, I see the following pattern:
For example, when repeatedly issuing “vol +”-commands, I see that a volume change is only applied every second/third time and then with 2 - 3% mostly.

Nevertheless, there is a discrepance between the volume percentage reported to mpd-clients (mpc / ncmpc) and the volume which is indeed set in alsamixer.

From my point of view, I consider this a bug. However, no error messages or warnings are logged in the mopidy log when controlling the volume.
Any hints?