If an enemy army is in its movement animation loop it can't be attacked.
Shown here in this Let's Play...
at the 14 minute mark.
In .86, the animation loop caused by getting stuck on terrain also seems to cause major slowdowns when you have them on screen. Of course this is only noticeable in large map, late games when the things are already chugging.
Yeah I noticed this glitch too, it's only been an issue for me in the new beta though
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