When placing stairs in certain ways some of the stairs get messed up. 

How to reproduce:

Place a stair in any direction.

Place another stair right next to it to create an L formation in the top part.

Place another stair to the opposing direction of the first stair was placed.

If done correctly, the stair will appear as a corner stair when it shouldn't be.

This was done on a server that runs on 1.13.2, and a local world, in 1.15.3 pre-release and 1.15.6 pre-release. This can be very annoying when building. This also allows for other weird things to occur, as can be seen on the third attachment.