Could have just had Oak catching Celebi in the movie and him just having forgot about it over the years and with Ivy coming into contact with it and the ball just being defunct and old, which is why it wouldn't open, would be a good callback to Oak forgetting things like his grandson's name and various other things, then you can bring it up later with Kurt figuring it out, and Oak/Ash deciding to release it since its been trapped for so long.
