Unfortunately, this happens on other airlines as well. It's just a byproduct of a not-quite-perfect-integration of partner reward tickets and systems.
It really is hard to say who is to blame and thus who's responsible to fix the problem.
What I usually try and do to avoid this situations is make sure I have the ticket numbers for the final ticketed itinerary (that's often the reason for a partner canceling a segment -- they don't receive the ticket number from the issuing airline by some deadline) as well as the confirmation codes for the partner airline flights, if they are different. I then try to make sure the reservation looks OK on the partner airline's website, using either the ticket # (preferable) or confirmation code. I think that's all one can really do. If the partner just decides to cancel a segment, there's not much that can be done, certainly not by the reservation agents and even their supervisors.