building help

Oh how annoying lol! Yeah I don't know, either. I also don't build stairs like that. They look great, by the way . Have you tried putting a railing up to see if that works? 

I am not sure if this link will help you but I will post it in case is does help you or another simmer 

https://www.reddit.com/r/Sims4/comments/q9yuae/how_to_remove_this_extra_floor_tile_when_building/

Did you find a resolution to this? I've never built the staircase the way you have so I wouldn't know too much, but looks an interesting technique if it does work :)