As the title says, as of the 1.42 update, the muzzle flash animation for secondary weapons (machine guns) on armour is always on.
See video capture I did here that shows the issue on a Marshall https://youtu.be/mdJwBzSInLk
I've observed this in the single player campaign final mission "Game Over". Have witnessed it on several different armoured vehicles (I think it also happens on the Panther).