It seems that Malphas has his co-op outline misaligned in co-op. He also appears too high on character selection screen. I haven't unlocked any other "big" characters yet so can't confirm if it's the same for them.
Also, I noticed that whenever the player gets any sort of transformation, even if it's just from an item, for instance, getting the Bone evolution from super box or Golden Finger, the outline is still visible on top of the transformation sprite.
Platform: Xbox Series X
Version: 1.12.107