The bug

Shulkers will not move when they are pushed by a piston into a non-solid block like tripwire or a button. Before 1.11 you could push shulkers into any block which makes it likely that this is supposed to fix that. But as non-solid blocks have no collision box but the piston head does you would expect the shulker to be pushed.

Expected behavior

Shulkers are always pushed into non-solid blocks and then teleport away if there is a place they can teleport to, if not they remain at the position of the non-solid block.

How to reproduce

1. Move high enough to make sure the shulker cannot teleport away
2. Place a piston and a solid block in front below to place the shulker
3. Encase every solid full block with for example glass
4. Place a shulker in front of the piston
5. Place a non-solid block like tripwire two blocks in front of the piston
6. Power the piston
   → The shulker remains at its position