Mobs going through nether portals on simulation distance 6 and up disappear.

Any mob that goes through a nether portal in the overworld doesn't appear in ther nether when running on simulation distance of 6 and up. It works fine on simulation distance 4, but once you change to simulation distance of 6 and up, no mobs appear. This is a very large bug for any mode of mob transport and mob farming too. I've tested this thoroughly with almost every mob, and none of them appear in the nether. I have no idea where they're going or why they are being deleted from existence. Please fix this as soon as possible. This affects 1.16.10.02 and all versions above it.