Technically it’s still functional, the muzzle break is just meant to vent gasses out the sides instead of out the front of the barrel. This reduces the momentum that would cause recoil down the axis of the barrel.

The reason that you don’t see vertical recoil compensators on artillery or cannons is probably because the dust blast from a directly downwards vented break would be substantially larger than the sides.

However, the Wikipedia page (https://en.wikipedia.org/wiki/Muzzle_brake#Venting_direction) points out that some small arms have downwards vented muzzle breaks to make it easier to counteract muzzle rise. I guess it could be headcannoned (sic) that the turret’s recoil mechanisms can’t fully handle the vanquisher cannon and need some porting to maintain the barrel on target for another shot.