@Andromedean This thread is from a while ago, and it may very well have been during some flooring issues that have gone on over the last year. At this point, since it was never responded to, it's a bit hard to confirm.
I believe that the reason I linked to the other thread was for some things that can be tried to work around some flooring issues that had been going on. It also had information on how re-create a floor or ceiling if yours has been deleted. So I would guess that's why I mentioned checking the other thread.
I see one mention of interest in having the flooring on the second floor in order to be able to place stairs, and aside from that I only read mentions of the poster having issues with a disappearing floor on the second floor, not stairs. True, I'm not fully understanding the issue, as there have been no discussions or further clarifications. You're welcome to advise on the issue and would like to help.