Here we go again. I now got to make a couple of videos that shows the problem described.
I have used Roon several times between the first post and this post, without the problem occurring. It comes back randomly and disappears when restarting the Roon core on my iMac Pro. See the two videos in this shared album:
The same thing happens when I adjust the volume manually using the volume knob on my T+A P 3000 HV preamp.