Minecraft 1.12.2
Forge 14.23.5.2854
DominionLib 1.0.4
Changeling 1.2

When you are morphed as a Shulker, you are unable to teleport, even when forced using commands (/tp, /teleport) or external programs. This is because those commands currently only update your actual position, and not your fixed position, causing you to teleport back one tick later.

Workaround: Demorph before teleporting.

Solution: /tp and /teleport on a Shulker morph should update your fixed location in addition to your actual position, but still aligning with the block grid.