Sounds like the same issue I reported here
Which I already tried and confirmed it works so, why you don't try to give the extra details I asked, like for example the exact sequence of operations you are doing or, perhaps, if it happens only after landing ?
Because, as I've said already, you just said "operate stairs" doesn't work so, lacking any better detail, I just went at the same airport, using the same airplane, and the same profile you posted, at the exact same gate and just did that: "operate stairs", and it worked at the first try, both stairs moved to the airplane and stayed there, as expected.