When a minecart with a shulker runs over an activator rail, the shulker is dispensed in a location where the shulker isn't able to attach. They exit just the same as any other mob.

When this occurs, the shulker starts to search for a valid block to teleport to and, especially when there isn't one or there are very few, creates lag.

I would expect a check for a valid block a shulker can stand on when exiting a minecart would occur. If none exists, the shulker should stay in the minecart.

This is a parity issue.