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.
Temporary fix: Break the portal in the nether, use a piston to push the shulker into the obsidian frame and relight the portal.