Stairs let you walk diagonally into 1 block gap from the lower step, disregarding the solid block

If there is a block gap adjacent to the stairs, you can move from the lower step of the stairs to that gap, disregarding if there is a solid block over it.

 

Attached are the images of the setup where it happened, I moved diagonally, hugging the wall to the left, causing my character to go into the coal ore and start block drowning.

 

In Screenshot (10) you can see my character standing on the ledge of the gap