Nvm, sorta jumped the gun with the question. Haven't looked at the source, but by inspection it seems the ComboBox's content view covers the underlying mouse area that is used to access the menu.

Setting the rightPadding and leftPadding of the ComboBox appropriately uncovers the clickable region on that side of the ComboBox, so, for example, setting rightPadding to 10 gives a 10px wide clickable region on the right, and likewise for the left.

Unplugging the tablet does not help. You have to at least temporarily disable the driver.
The issue is described in detail here: https://bugreports.qt.io/browse/QTBUG-47548

You can stop the driver like this: net stop WTabletServicePro
And restart the driver with this command: net start WTabletServicePro