Just found the answer. Reverting to <Unknown Subtype> occurs when you have a part with the subtype you are editing nominated. From what I can tell, if you are going to edit a subtype, make sure all other parts you are editing are closed.
Michael Wilson
ArchiCAD 9 - 22 (6021 AUS Full)
Win7 | 64 bit | 8Gb RAM | Intel i7 3.40 Ghz