The bug in the Furniture Library Editor is more subtle than that: the staircase cut out shape is reset to its default value when multiple models with a different shape are modified. When you edit only one model, it's ok.
But anyway, it's fixed now (like many other small bugs I found in the Furniture Library Editor). I can send you a beta version if you want.

[Edit: Sorry, I should have read your message better since it spoke about the doorOrWindowCutOutShape# property, and not about staircaseCutOutShape# property. I was troubled by the fact I fixed a bug about the later one recently.
As the doorOrWindowCutOutShape# property is new, it's not managed by the Furniture Library Editor, but the current beta version of the Editor does manage it]