I've this issue repeatedly – there seems to be a mouseover bug where sometimes the combat preview (when mouseover enemy army w/ an army selected) will get triggered but won't disappear; presumably because the mouse up event doesn't get recognized, or something. 

In my experience, this'll leave garbage on the map (combat preview graphics, etc), and the armies disappear (both the army you had selected, and the one you were targeting) b/c the game hides them when showing you a combat preview. And this does not disappear after the turn ends so the units will just stay hidden; I don't think it persists across game saves though (or at least, I hope it doesn't; if it does, the devs are doing something seriously wrong with game state serialization lol)

Anyways, this seems to be fixable by selecting the same army that had caused this bug and re-mousing over the same enemy army that had been targeted, re-triggering the combat preview and then moving the mouse away so it can disappear properly.

If any of the devs are reading this, I'm almost certain that this is a bug where a mouseup event / focus change event / whatever will sometimes get missed (dropped frame? low framerate / high latency on an update loop somewhere?), and the code should be rearchitected so this can't happen.

To repro try running the game on shitty hardware / low framerate and try mousing on / off units to toggle the combat preview, which should trigger this if the bug is in fact framerate dependent