I'm using SW 2014 and it exhibits this repeatable, infuriating behavior:

1. Create a multibody part

2. Insert each body into individual parts

3. Change the appearance (part level) in each newly created part

4. Create assembly out of parts

5. Save assembly

Action #5 clears EVERY SINGLE PART-LEVEL APPEARANCE FROM ALL OF THE COMPONENT PARTS. This only occurs the first time you save the newly created assembly file.

This is just another in a decade-long trend of buggy or poorly implemented appearances. I wish Dessault would get it through their thick skulls that "appearances" isn't just a minor feature that no one cares about, it is the ONLY thing users care about. The ONLY thing a CAD system is used for is to turn ideas, geometry, etc, into PICTURES for communication to others VISUALLY. You can't say "our CAD system is good except for the way it visually communicates information."