RoonBridge is using the first volume + mute controls it can find on the device. That covers a lot of cases, but apparently not this one. I think the correct control is, paradoxically, the one called “Digital”. Not sure if it’s actually a digital adjustment or not–it looks like the driver or ALSA is making a bit of a mess, so something might just be confused about the names.
It would be good to understand more about why this device is laid out like this in the ALSA mixer, and if it is totally necessary for it to be that way. There’s nothing technically impossible about having Roon grab an alternate slider instead of the first one (which is the main one 99% of the time), but I’m not sure how that would look like on the settings screen.
It should be noted that the official IQaudIO images handle this tricky situation properly with zero configuration. If I had this device, I would strongly consider using it instead of RoonBridge.