There's nothing fishy and UA/AA do not "buy" inventory on NH and JL. In fact, there's a Joint venture and the entire itinerary is metal neutral.

It's just an instance of there being a disconnect between the two airlines' CRS systems.

It's possible, since that this is MNL, is that there are certain fares for the local market that are not available when purchased with a US-originating itinerary. ITA is pricing an itinerary using these fares, possibly a conjunctive ticket, and when SHARES (or SABRE in AA's case) goes to ticket it, it's not available because what ITA built violated the fare rules.

I forgot to mention, in the case of virtually every airline and OTA, the fares and itineraries are built by ITA, not SHARES, SABRE, AMADEUS, etc. Sometimes there's a disconnect between what ITA thinks the fare rules are and what is actually in the airline's CRS. Unfortunately there's nothing that airlines or OTA's can do about this because ITA is pretty much a black box.