The bug

Regular entities treat boats and shulkers as solid. You collide with them just like blocks. Boats collide with themselves in a similar fashion – they can stack on top of each other. However, boats do not collide with shulkers. They pass right through instead of landing on top like every other entity in the game.

How to reproduce

Since shulkers can enter boats, it's important that you fill the boat with passengers before attempting this.

Method 1:
1. Create this setup:

2. Break the support block, allowing the boat to fall onto the shulker
→  It clips through the shulker and lands on the ground

Method 2:
1. Spawn a shulker
2. Place a boat, and spawn a mob to ride in it
3. Begin riding the boat, and drive towards the shulker
→  You pass right through it as though it weren't even there