If you activate an elytra and get into flying mode, then when you land on a either a shulker or a boat you will begin to crawl. There is nothing above you to prevent you from standing up, so this is unlike other crawling behaviour.
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violine1101
In MC-154543 you are underneath the hitbox of the shulker / boat, so MC-154543 is kind of consistent with the crawling behavior associated with normal blocks and might even be intended. In this case there is nothing above the player, so it makes less sense that they should crawl. Maybe they are the same bug, but in this case the Elytra hitbox alters the behavior further.
This does indeed look like MC-154543
What exactly is the difference between this ticket and MC-154543? They look the same to me.