Java: Version 8 Update 171
In 1.13 server
As shown in the image I attached, I think the game is taking the sign as a block.
I was using the new feature to swim in a cube of water on a bunch of signs.
However, when I drop through the signs, my character keep in this pose.
When I walk the the edge of those signs, my character return to the standing post.
MC-125774 The player keeps its 1 block height of swimming animation when going out of water