I'm not sure whether this is the same problem as I have not experienced it with stairs yet but this started happening with a ladder in a house that used to work fine. I'm not sure when it started as I have only rotated back to this house since the [RL] EP released. It is very simmilar to a problem that has for a long time happened with doors, in that they suddenly aren't routable. As with doors, what temporarily fixes it for me is to enter build mode, click on the 'move' icon on the ladder (or door) and then hit ESC or 'undo' to set it back where it was. When I re-enter live mode, the ladder works again but only for a couple of 'uses' before it is un-routable again. Not sure what the trigger is. Not easy to play test as sometimes it does not occur at all during a play session.