The delay hinges on US gov approval of AMRAAM integration on FA-50 Block 2.
RTX/Raytheon, producer of PhantomStrike AESA radar (FA-50PL, FA-50MY) and AIM-120 is eager to get that approved.
LockheedMartin otherwise don‘t want a more capable competitor to their F-16V.
The outcome is uncertain but the US are stalling for time and delaying a decision.
During KF-21 development they first refused an integration of AMRAAM & AIM-9X Sidewinder. After Koreans signed a deal with MBDA Meteor and Diehl Defence IRIS-T the US suddenly "reassessed" and approved both missiles for integration.
Not sure if it was an overpromising of KAI or AIM-120 integration was never promised as 100% certainty in the contracts with Poland and Malaysia. For a fact, ROKAF doesn’t need BVR on FA-50 or want to pay/share the absurdly high integration costs. Expensive AIM-120s are reserved for superior fighter jets F-35, F-15k, KF-16 & KF-21.
This Polish opinion piece sums up the situation pretty well:
In a parliamentary speech, Deputy Minister of Defence Cezary Tomczyk criticized the process of purchasing FA-50 aircraft, particularly the lack of weapons involved in that procurement. But was he right? Let’s look at the facts.
defence24.com