When you place a stair (any) it has his correct hitbox, but after putting an other one beside it to make a corner and then breake it, it start to show a different hitbox.
After this, every single stair that you place become glitched.
(not alwais, but if you test some times you will see that the hitbox start to do strange things)

1st image: normal stair
2nd image: glitched stair hitbox (''air'' zone)
3rd image: glitched stair hitbox (''stair'' zone)