Roon doesn't recognise scrolling gestures with DEX

I’m using my Samsung S20 phone with DEX hooked up to a monitor and MS media keyboard to control Roon.
The trackpad on the keyboard works fine as a pointer but Roon does not recognise scrolling gestures so can’t navigate albums/tracks etc.
Trackpad scrolling works fine with internet apps etc under DEX, so it doesn’t appear to be a hardware or DEX issue.
To be clear: using a single finger on the keyboard trackpad to move the pointer and click selections works fine. Using two fingers on the trackpad to initiate scrolling action left/right up/down is not recognised by Roon.

With more experimenting it seems I can scroll if I click the trackpad and hold down while using a scrolling gesture.
Weirdly it seems to be hit and miss and clumsy.

Seems I need to click with one finger, hold down and scroll with two. Clicking down with two fingers to simplify the action doesn’t work.