Since you had trouble shifting up to the 50T, did you end up using the small ring with the smaller cassette cogs (11, 12, 13T) a lot? The flexing around smaller cogs can wear a chain out faster, but 700 miles is still pretty hard to believe! I bet what happened was that the chain was marginal, and they used a chain-checker tool, which are known to "fail" chains that still have some life left in them.

Next time you take a bike in for derailleur adjustments, you (and anyone else who's had shifting trouble) should ask them to demonstrate the shifting on the stand before taking it home. It shouldn't take more than a minute, and both parties can be satisfied that the bike shifted through all gear combinations without issue.