For whatever reasons, bullsquids have a weird melee hitbox. This can be noticed in on a rail, where after a bullsquid teleports in when you put the plug in for the bridge, it can hit you through the wall. Also in residue processing, in the basement area near the switch you flip near the revolver, if you stand above the bullsquid, it still melees and does damage to you.
Noticed same issue in Blast pit chapter. When activating generator, bullsquid in one of the controll rooms is also able to hit you through wall.