Why do the smallest displacements (and minion collisions) make some champions a bore

Was playing anivia and trundle and I could not move through minions after using a displacement on myself. I was literally boxed out of large gaps between the minions by some weird power of having a created terrain almost on my champion model. Seems like a bug where ceratin champs have had pathing upgrades and some haven't when it comes to displacements and created terrain.