This has been somewhat discussed in a couple of other threads, but I didn't see exactly how airlines and countries are handling APIS information in the case of a dual national traveling between the two countries of their citizenships. The concern is that traveling from Country "A" (which does NOT accept dual nationality), checking in with the airline with country "B"'s passport (the destination country, which doesn't care about dual nationality), the airline might share the check-in data with the departure country via APIS. Concrete example: Spanish-US dual national. Spain does not accept this dual nationality. The US doesn't care. Check in with an EU airline for a flight Spain-US. The "easy" thing to do (so that the airline has no doubt that the passenger has landing rights in the US, without a visa or ESTA) would be to check in with the US passport. (And then show the Spanish passport at Spain's exit passport control checkpoint). But, will the airline send APIS data to Spain (the country from which the flight departs)? .. in which case there's some risk that Spain (which doesn't accept dual nationality) might get a bit upset (if it realized that this one physical person has two different passport identities).
On the other hand, the passenger could check in for the flight from Spain to the US using their Spanish passport, and then show the airline check-in staff (surely they'd be unable to use the kiosks and online check-in) their US passport, so that the airline would be satisfied about landing rights. IFF the airline sent APIS data to Spain, then Spain would just see a Spanish citizen flying out of Spain - no problem. The US would get APIS data about an arriving Spanish citizen, which might result in a question at US immigration, but as the US doesn't object to this, it shouldn't be a big deal. (It might, however, prevent using the self-service kiosks and Global Entry, which would be an unwelcome pain).
So, does the ever-more-common APIS (all the countries these days want all the information they can gobble, from everywhere, as early as possible) create any new complexities for dual nationals traveling between a country of their nationality which does not accept dual nationality, and the other country of their nationality?
What is the most convenient (still able to use automated check-in and especially immigration/ US Global Entry kiosks) strategy of which passport to use when, while ensuring that the country of citizenship which does not accept dual nationality doesn't get upset?