Interestingly, I had this issue at PHL on a DUB-PHL-NCO Buisness class trip.
At the virtually empty AA lounge at PHL, the couple in front of us at entrance who had been on the same flight as us were being denied access as the pho-mco was not eligible for access.
I had to explain to the gate dragon, what the rules were. She then let us all in. It may be the couple hadn't explained properly, but it's confusing for Europeans not to gave lounge access when travelling in business/first