Child passport - birth certificate (non UK)

[obormot]

I recently applied for UK passport for my 11 years old kid. He was born abroad, before the family came to UK 6 years ago, and there are no british ancestors.
We are EU citizens, I became naturalised UK citizen in December, and the child got the registration certificate (the further has settled status).
Today I received a letter form the passports agency asking to send to them child's original complete birth certificate with parents names included.
I thought we do not need to send birth certificate?
Did I miss something?
Is it something that happens often (them asking for birth certificate)?
Do we need to also send our marriage certificate "just in case"? We were married before the kid was born, and continue to be so. Thus, there are no "court orders", etc.

P.S. This <censored> process never ends! My naturalisation certificate misprinted my birth place and I had to send it for replacement; my 17 years old is being asked for passport interview; my non-EU mother apparently will have to replace her EU family member BRC for a "settled" one, and after that she will have to send that new card to settled status office to update her documents...

[obormot]

I sent them kid's birth certificate yesterday (delivered this morning), and just got the email that his passport was approved.
Still - I went back to online form and checked what documents they asked me initially - there was no mentioning of birth certificate!