If you look under the flipped over stair (which is using while crossing the stairs) inside water, you'll see, that the part of it is flasing. You can find it easially using the Debug world. It works with every stair.
Environment
Java Edition. Windows 7 1.13 pre-10 Debug world No texturpacks, shaders or anything.
it has since when waterlogged feature added
poop english 😞