Totally Immune Units

[Melvish]

I had something similar once in a game with France in a battle against Savoy. It turn out that a cavalry unit was half garrisoned into a house and the other half came chasing my cannons. They were "untargetable" by mouse click but i mange to engage them by moving a unit in the same spot.

I had also a battle where i was playing the UK against the Huron where i had all 3 of my dragoon units stuck under a hill and totally unusable.

I'm positive that what you experienced is a bug and is related to one of the examples i've described. Feel free to report it in the bug report thread.