This bug is one of the known issues listed in the Issue Status - PC - Thread. A temporary fix is to have a foundation under the stairs that you want to add (though depending on the look you are going for, this could be less then ideal). Whatever the problem is, I haven’t been able to figure out a fix myself (I spent hours in the dev kit trying different things). Hopefully Funcom is having more success then I did. Anyhoo, just thought I would let you know 