@Mstybl95 Ok, that got me far enough that I was able to make a fix on my own! I did as you suggested, but deleting the floor the second time caused the floor to disappear in the adjacent room as well-- the floor of the second-story outdoor walkway, on the other side of the wall from the room with the stairs. Here's my fix, just in case anyone else has this particular problem, I know a lot of people struggle when building with stairs. (There were also a few other oddities I ran into that others might find interesting or have seen before? I don't build much and always happy to learn something.)
Fences in place on second floor
https://i.ibb.co/fnxyn9T/06-02-21-3-58-40-PM.png
I had to draw a fence on the first floor as well. This runs directly underneath and parallel to the second-floor wall with all the windows separating the stairs room from the outdoor walkway.
https://i.ibb.co/QmNCd7P/06-02-21-4-07-52-PM.png
The fence happens to run directly across the bottom of the lowest staircase. Note that the stairs have now been cut in half and are floating in midair.
https://i.ibb.co/fGD4Wws/06-02-21-4-08-21-PM.png
https://i.ibb.co/F6Sd1gK/06-02-21-4-08-26-PM.png
Deleted one section of flooring, all is well.
https://i.ibb.co/wwMMrSZ/06-02-21-4-07-56-PM.png
Removed fence, deleted next section of flooring, all is well.
https://i.ibb.co/VL47KMK/06-02-21-4-09-05-PM.png
Removed fence, one section of flooring came back.
https://i.ibb.co/wKCycJ6/06-02-21-4-09-15-PM.png
Deleted that section of flooring. Outdoor walkway flooring in the next room remains intact.
https://i.ibb.co/zHLGRQx/06-02-21-4-09-24-PM.png
But the lowest staircase disappeared completely. (The first-floor fence remained; I removed it manually before I took the screenshot.)
https://i.ibb.co/zR1XN5s/06-02-21-4-10-06-PM.png
I built another staircase. Oddly, it is in exactly the same place as the old one, same height, but it extends one tile less far into the room. It must be a bit steeper than the old staircase.
https://i.ibb.co/XpNjkKz/06-02-21-4-10-29-PM.png
I don't really mind that so much, but it did make me wonder, have I stumbled upon some complicated way to adjust the slope of stairs?? (steepness/shallowness)
I also wish I could delete those back stair rails that are drawn on the wall, or the weird floating lines above the stairs, but I think this will have to be good enough. Moving on with life.
https://i.ibb.co/Nmzk4MG/06-02-21-4-12-29-PM.png
https://i.ibb.co/9318QSp/06-02-21-4-12-43-PM.png