Nice to hear from you again.
A certificate of birth is an important document and Standesamt won't issue it without knowing who exactly the parents of the child are. They want to avoid ending up with false information in the certificate. In order to check your identity they need a number of documents including your passport. That's the reason why it's lawful for Standesamt to just give you a birth registration if you can't provide a passport (and other required documents).
I'm not sure about the verification of your passport, but since it's important to check your identity I assume they have to verify it first. Probably there is a number of countries which passports are not accepted unconditionally by German authorities without further investigation.