This isn't unique to JX either. Here is an example of the same issue I can reproduce on a QR J award:
New flight options. Adding onward segment shows only a difference in taxes owed:
Summary prior to submission of change only shows the added leg and not the full itinerary like on the previous page:
Exchange summary showing I should be swapping a QR U/saver J priced award, but the flight summary only shows the "added" leg.
If I followed through I've no doubt it'd have turned out just like
boneofthet ticket with the initial J portion dropped. In my case I knew this isn't how it should look, so I stopped and left it alone. Hopefully AS can work to get the OP's ticket back somehow, but it sounds like they think it's user error vs something wonky going on with their system so I doubt it.