Strange volume behaviour

I have a strange case when trying to adjust the volume on a roon iphone endpoint.

I have a short screenshot video that show what happens.

At some volume levels - when I click + to increase the volume by 1 - the volume increases - and then returns to the old level.

I have a smilar case where I reduce by 1 - and it drops 2.

I was actually trying to investigate whether I has a bug in my home assistant code. But this happens without home assistant running.