Umberto, congrats on the release today with some great improvements I know we have been waiting on!
We found the problem, and it seems to happen only when there's no passenger bus coming, will be fixed in the next update.
I am wondering if there has been any progress made on this front? A number of updates has occurred since this was noted and I'm still seeing issues with this.
I am finding now the rear stairs don't appear to want to move up to the plane at all. Both doors open on their own, though! Running the latest update - 2.4.0 - though this same issue was present in the previous version.
Edit: Since the latest update (SU12 Navdata fixed on), it seems the 'Disable rear staircase...' option has no effect on this issue, the rear staircase does not animate and move up to the rear door. When requesting boarding, crew, pax etc will use the staircase that hasn't moved and walk through the air to the aircraft. Log attached.