Custom Mob Suffocating?

I guess this counts as a bug? I made a custom mob the other day with a custom model. Thew model is less than a block tall, as is the custom mob's hitbox. However, when I test it in-game, if the mob goes into a space that's just one block tall (which it should be able to do) it suffocates. I have no idea whatsoever why this is occurring. There's probably an easy fix. Any help here? What am I missing?