When I am standing on stairs and open a trapdoor above me, I start to swim. I haven't tested this anywhere else but the picture is the exact circumstance.
Environment
I think my OS is Windows 10, I'm not sure.
The Minecraft version is 1.14, I'm not sure if that's the same thing as Java Version.
This happened on a server run by my friend, I think he pays for it though.
Duplicate of MC-147612