This bug is a bit of a handicap:
If too many (more than a couple dozen) separate unit stacks are present in a region, the unit icon ribbon display widget at the bottom of the screen gets confused.
There is only one stack displayed, and no header tab.
So, actions that depend on those header tabs (such as trying to select/merge units in the region, or target stack(s) as destination from other regions) become impossible.
For some reason, some actions on the one stack displayed (such as dragging it to a new region, then cancelling the move order; or merging units in that stack) cause the unit icon ribbon to "reset".
I suppose we could consider that a bug work-around ...