We are unable to reproduce this with the Linn Majik DSM unit we have in our QA lab.
The next time that you see this behavior occur, please post in this thread with the local time of the incident. We will then be able to enable diagnostics on your account and look at the logs from you Roon Core in the time around the event.
Some further questions:
A) Does this behavior happen every time you attempt to use the volume control?
B) When you use the IR remote to control the volume, does the volume level update in Roon in-tandem?
C) If you tap the volume widget in the bottom right corner of the Roon app, then tap the “gears” icon, then the “Device Setup” link, what is shown next to the “Volume Control” label? If it isn’t already, please set this to “Device Volume”.