That would have been my second guess after a computer bot.
I had the issue you described with a set of Back to the Future years ago. I think I got the first film and two discs of the third film. The second film was not included.
I wonder if there are bots that people have programmed just to try random codes in the hope of scoring once in a while?
I'd imagine if you have enough Disney codes there must be some sort of possible pattern that a computer could figure out.