Sorry if this is off topic, but it might be related.
I don't recall this happening in .77, so it might be new in .086. If I have an army, and select one of the members (to examine its stats, for example), and then forget that I have it selected, and right click on a destination for what I think is the whole stack, sometime that selected unit moves by itself, and sometimes nothing happens (always if the unit has no movement left, and sometimes even if it does). Once this happens, the selected unit is removed from the army it now shares the tile with, and I haven't found a way to put it back into the army without actually moving it and the army to a different tile.
Has anyone else seen this?