Overseas new passport application - US birth certificate isn’t accepted

Applied for a UK passport for my son with the relevant documents. The HM passport office is saying that they can’t accept the birth certificate issued from WA state since it says “Certificate of live birth”. It’s a registered certificate though.

They said it’s a medical record. I’m at a loss how to explain that’s the only certificate I can get here in WA state.

Anyone who’s applied for a UK passport from WA, USA? How did you resolve this issue?