So in 1.12.2, I'm playing around with a custom entity with a model exported from Tabula. This entity extends a custom entity class which itself extends EntityCreature, but to put it shortly it's heavily derived from the Ender Dragon.
That said, in its onUpdate method, when I call super.onUpdate, the entity just falls through the ground and into the void. I call setsize() in my constructor, so I'm not sure what it could be.
As always, one always finds the solution on their own just after posting. Keeping this up for any future people who have the same problem
Because the Ender Dragon has custom hitboxes with multiple components, its bounding box is set to noclip = true. Disable that, and the mob works as normal. However, this means that the multiple hitboxes, usually inside the mob, will not work correctly if the mob's bounding box is too big. So just make it small.