There is one place I find in the game where the tabs on the bottom panel don't behave as expected.
When I have a fleet at sea ... Suppose I want to conduct a landing of ground forces.
(1) Drag a single unit to a region from the fleet.
(2) You will now have two tabs (the fleet and the landing force).
(3) If you try selecting additional ground units from the fleet and dragging them on to the landing force's tab, nothing will happen. If you try the same operation for forces on land, the dragged units will be transferred to the target tab's force.
This seems a little bit inconsistent to me.