The problem is likely linked to the fix that has been posted to fix the crash when stairs starts to move, which is only a temporary workaround.
This has the side effect on the animation of the rear stairs, it's only a visual issue, because you'll notice when the stairs retracts, it snaps in the correct position for a moment, which seems to indicate the real crashin problem the fix only temporary cured, is related to the communication between GSX and the sim, more likely is a timing issue, that's why the fix doesn't seem to work for everybody.
As already explained in other posts, since it doesn't seem like an obvious mistake that can be fixed like all the other fixes that has already been posted minutes or hours at most after they have been first reported, I need to wait to discuss it with the other GSX programmer (who is the *main* GSX programmer), starting tomorrow.