This has been discussed many times and, it's not really a bug, it's more something that would require a change in design.
The CONDITIONAL embedded stair cannot be detected on arrival, because the choice to create (or not) stairs is done depending on the variable controlling the condition, but the variable won't be set until after you park and extend the airstairs, but by that time, the decision to create stairs has already been made when you select the gate on taxi (or even before the arrival).
So, ideally, we should have a way to announce your *future* intention to use the embedded stairs when the gate is selected, which is just not there, but we are planning this.
The closest thing to that, is to create an airline-specific .CFG file where the Conditional staircase is replaced with a fixed condition (just type 1 in the xml condition field, so it will always be True), to simulate an airline-specific policy of always use the airplane own stairs.