At this point, we need a step-by-step workflow of your process.  The nighty is a dynamic cloth fitted for LF2.  If your base model is an LF2 figure, as you claim, there is no need to use the Fitting Room.  This is content for the Cloth Room.

DeeceyArt has identified the figure as an LF1 model. Before we can assist you, we need to verify this information to avoid confusion.  We have been down this road before.  

To clarify how clothing is saved after refitting in the Fitting Room, Create either the prop or conforming clothing and exit the Fitting Room.  The Pose Room is where you make your final save to the Library.