pavucontrol doesn't work properly insida a ThinLinc session for some reason. If you drag the device volume control then it will quickly stop tracking your mouse movements, forcing you to grab it repeatedly. Stream volumes work fine though.
I suspect this might be some broken feedback where the volume change ping pongs with something further down the chain, causing an update event to be sent to pavucontrol. This in turn makes it abort the current user drag.
Verified that this is still a problem with Xfce / Pavucontrol on Fedora 28 platform.