When entities (zombified piglins, for example) are sent through a nether portal from the nether to the overworld, if a player is loading the chunks on the other side of the portal, then they move normally and don't let their hitboxes intersect. However, when portal itself is the only thing loading the chunks, the piglins do let their hitboxes intersect.

Specifically, this breaks simple portal-based mob farms, like ianxofour's design.