Thanks, that helped. I discovered a problem with property caching and querying an unloaded part document. This will be logged and fixed. There is a simple one-time workaround:
1) Open the drawing.
2) Open the part.
3) In Part: Rebuild all.
4) Switch to drawing. It is updated now.
5) Save drawing and part.
After this, I do not believe you will see this problem.
Katrin Grunawalt
Autodesk Developer