I don't know the cause, but as a 'solution' I'd recommend skipping that third sequence. Just use the cuts in their original sequence.
I don't think skipping the third sequence works for me. I'm usually editing long performance pieces with multiple hour long multicam sequences that need to be put together in a composite, either showing the full concert or just highlights. Once I get an act edited I like to work with it as a single entity, combining it with other acts.
Incidentally, I just checked out my scenario on CS6 and it works fine, just like CS5.
I think I've found my answer. The problem occurred because I was creating the multicam stacked sequence the CS5/6 way. If I use the CC way with the "Create multi-cam source sequence" method, the resulting edit can be copied. I think there's still some bug but as long as I do the multicam edit the "new way" I should be OK. I had tried the new way before and had problems but I think I can live with them. Especially now that I can tweak the audio-based synchronization to allow for time delay to the various cameras.