When summoning a sniffer (for example) it can be summoned outside the world border surprisingly, but the strange part is it can go through the invisible border located outside the world border, all the sniffer has to do is walk through the border or you can just summon the sniffer next to the invisible border and it will glitch through.

How to reproduce

Start by summoning a sniffer outside the world border, you can use the following command to teleport outside the world border:

/tp 29999999 256 29999999

Then wait for it to walk through the invisible border, this won't take much time since the border is very close.

You will realize it can indeed go through the border.

Observations: Sniffer and many other mobs can go through any border including the invisible world border.

Expectations: No mob can cross any border.