Shulker riding minecart can be "outside" of the minecart when looking from the right camera angle. This issue also happens when is shulker riding minecart with chest, hopper, spawner or furnace. It can also cause z-fighting
Can confirm in 26.4 Snapshot 1 and 26.3. It is only one side of the Minecart (one of the long sides) that the shulker protrudes from; all other sides, including the bottom, are fine. This happens with both the summon command from Avoma and shulkers getting into minecarts normally.
Can confirm in 1.17.