Happened to my friend, flew Delta JFK > SLC > Hawaii (not sure the airport code).
They had a negative covid test but learned at the airport that hawaii required it be from an authorized location (Not the free ones the city of NY gives) so they took the test in T4 at JFK at 200 a pop (3 people) and boarded the flight to SLC with hopes that the test results would hit their phones before they boarded the plane in SLC for Hawaii. It didn't she and her family boarded the flight to hwaii anyway, not sure if Delta put up a fight, but when she arrived in Hawaii she had her results on her phone.