I found tracks disappearing in certain parts of a multicamera track that seems to be caused by using Event Heal to combine the multicamera track segments created by Vegas going into multitrack mode. For example, if there's just one camera for a few seconds at the start, combining that segment with the next with multiple cameras drops all but that first camera.
But after upgrading to V19 648 it seems the Expand back to separate tracks is always grayed out.
Regards,
Richard