Just thought I'll share my personal experience.
I believe the quality of film matters. I recently wrote in this forum about some problems that I had with Shanghai GP3. It subsequently turned out that the problem was caused by poor storage of the film. You can see the thread below.
I bought a fresh stock of Shanghai GP3 and kept it in my camera for about a week. Camera was kept in my car. I live in thr tropics so it can get pretty hot. Anyway, here's two shots from the same roll:
This one turned out ok
Whereas in this picture, you can see the number imprinted:
Both pics were taken within an hour apart.
All I can conclude is that Shanghai 120 films do not store well in hot, humid weathers, whether they've been partially shot, or sealed. From what I've read on the internet, and from comments from other Shanghai GP3 users, this is a common problem with the film.
I do not have the same problem with 35mm film though, they frequently sit in my cameras for weeks.