Shulkers appear to be inside a block after removing the boat they are in, they do not take suffocation damage while inside the block, but this can lead to transporting issues, making this probably impossible to transfer shulkers for an advancement, this bug happens to all other boats.
Open up Minecraft 1.13, spawn in a shulker and place a boat next to them to let them in the boat, then remove the boat and see the results, sometimes this can be successful for shulkers not going inside the block but this mostly fails to attempt.
Here are the 2 comparisons that show this issue.
Shulker in boat before removing:
After removing the boat, inside a block: