Whatever map you are on and whatever server online and either offline session, when you place a slave on your built floor near the ground they pass through the built floor building.
This seems like a bug that came with the new patch. I am experiencing the same.
Bit of an old bug actually, iv’e had this bug since the first patch after LS was introduced sadly.
I had to turn the feature off because every time my workers went to their stations they would just start falling through the floors on the ground floor and with that every time i placed someone on the ground floor they would just fall through the next time i went through that area.
Even further back still there was an issue to some degree even where i couldn’t place mounts on tiles that were placed on the ground or my mounts would just fall through after revisiting the area.
It’s not a newly introduced bug for sure , it’s just an old bug getting worse after every patch and becoming less and less random.