The title pretty much says it all. When a shulker is inside a water block, even if it is riding another entity and its AI is turned off, it instantly dismounts and tries to realign itself with the block grid.
See this video for reference: https://www.youtube.com/watch?v=5BsX63MNeSg
Entities dismount in water, NoAI bugs are considered WAI by default. Not a bug.