Shulker mobs will not go through portals while in the nether. This was tested in SMP and Singleplayer Minecraft 1.11.2 for PC.

How to replicate:

-Put a shulker in a boat
-Send the shulker to the nether via Overworld portal by breaking the boat
-Try to send it back to the Overworld via Nether portal in the nether by breaking the boat
-Shulker will only teleport on the block next to the nether portal.

Pushing the shulker into the portal with a piston also does not work.