The bug

See the attached vidcap. The shulker opens and doesn't got through the portal, I switch to survival long enough for it to open fully and shoot at me and it still doesn't go through the portal. This is not a portal cooldown issue. I've had this shulker there for many minutes.

How to reproduce

1. Download the attached world (MC-238658.zip)
2. Spawn a Shulker on the side of the blue wall closest to the Nether portal
3. Wait until the Shulker opens its shell
   → The Shulker travels through the portal
4. Enter the portal
5. Spawn around a dozen Shulkers inside the portal in the Nether
6. Return through the Portal
7. Find a Shulker attached to the blue wall and wait for it to open its shell
   → The Shulker doesn't travel through the portal