Ghastling leashed to mob dies by suffocation in a block

While playing survival offline, I began hatching a ghastling and had one camel leashed to a fence. When the ghastling hatched, I noticed that it moves very slowly, and moves so slowly when pulled by a lead that it can break the lead when the player walks at a normal pace. I then leashed the ghastling to my camel. I left the area for mining and came back. I observed my ghastling, which was levatating very low to the ground. It was one block stepped down from the camel and fence. It suddenly began taking damage at the same rate as suffocation. I walked over to it and pushed away from the block by walking and colliding with it, and that stopped the damage. I then quickly went online to enable screen recording. It then started taking damage again, so I pushed it away from the block again. It started taking damage once again and died.

Conjecture/Theory: ghastling movement is slowed down in all categories, rather than just its independent movement. This prevents the mob from being pulled by a lead and prevents the mob from moving out of a block during collision.