I've had that issue as well at top of stairs when doing a spiral type staircase and the top of stairs would stay open not allowing me to place a floor tile. This was months ago tho and I found out that it was due to what was right below that tile. The work around is that it worked if I had the walls at medium or tall height.
I even tried making a 1 sq tile to place at top of stairs to see if that'd work. Can't recall if that did, I assume it would not have.
But a screenshot would be good.
My previous issue from last year was resolved after a game update, so something in that update fixed my problem I had in July last year.