The Bug

When a shulker is the passenger of a leashed boat which is in the air, the shulker does not open their shell when peaking or attacking visually. They still do shoot their shulker bullets however, but this is done while their shell is closed. The 'Shulker opens' and 'Shulker closes' sounds also play.

How to Reproduce

1. Place a boat and summon a shulker. Then run /ride @n[type=minecraft:shulker] mount @n[type=minecraft:boat].
2. Leash the boat, and fly up with it in Creative. Also, run /effect give @s minecraft:resistance 60 9 true to protect yourself from shulker bullets.
3. Once high enough (around 20-30 blocks up), go into Survival mode (make sure the leash is still attached to the boat) and land on top of the shulker, so that it will aggro on you.
4. Note how the shulker shoots at you, but the shell does not visually open. Going into third-person view allows you to see this more clearly.