Almost the same thing happens with the animations from magic staves in player armies, but the pattern of occurrence is different. It seems to start happening randomly within a given battle, but once it starts it happens on all player-owned staff attacks for the rest of the battle.

I also see an anecdotal correlation between this bug and game crashes on the player turn after the combat ends, as if the misplaced animations were trashing something in memory that's usually not fatal but can be if you get unlucky.