Going into fly mode in a 1.5 block space (3rd person)

I was underwater (though it seems to be on land as well) and I went into third person and I was in a confined 1.5 block space, I would usually be forced to crouch, but whenever I went into fly mode it seemed as though the player's head was in the slab. It did not affect first person views, however. I attached visuals below.