I'm flying to Singapore on a BA-marketed but AA operated ticket - NYC to SIN through LHR. Leg one is AA operated, the rest is BA.
When I tried to check in online on BA, they said they couldn't verify my documents. Just tried to check in at JFK at the AA counter- they refused to check me in, saying I need a special embassy pass to visit Singapore. I've read through every US/Singapore embassy site I could find- no where does it say US citizens need any special pass, not even a VISA.

I have all the documents needed for Singapore entry based on the Safe travel website (The SG arrival card, the VTP, insurance, neg covid tests/vax cards and docs)

Has anyone else had a similar issue?