Re: Floor issue on top of customised stairs
If you want to keep the foundation, you can always put it back after, or create a duplicate lot using Save Lot To Library so you don't lose your current progress. And you don't have to keep the lack of foundation, I'm mostly asking in case this is "staircases being a bit weird about foundations or platforms".
ETA: customised stairs are supposed to remove tiles directly above them, but it seems unintentional that this is inconsistently applied and/or only affecting floor/ceiling tiles and not walls, nor should it be trying to delete the stairs whenever you try and place a wall. I can't tell from your image if it's only affecting the floor/ceiling tiles directly above the staircase or another floor up - but either way, something unintentional seems to be happening.
Hi! I'm SimmerStar4 (they/them). I'm not an expert on game issues, so don't take me as one! I'm just a Sims player who wants to try and help fellow players, and who finds game bugs and issues interesting.