App Engine > Subdomain > Add A, AAAA and CNAME record

Is it okey to add the A and AAAA records but not the CNAME record when adding a subdomain to App Engine?

Read the following SO threads, but couldn't find the answer:

https://stackoverflow.com/questions/55576228/custom-subdomain-for-google-app-engine-service?rq=2

https://stackoverflow.com/questions/58357461/are-google-app-engine-a-aaaa-records-fixed?rq=2

https://stackoverflow.com/questions/62917498/adding-google-managed-auto-renewing-ssl-custom-domain-with-cloudflare-dns?rq=2

https://stackoverflow.com/questions/75116753/url-not-found-adding-a-custom-domain-gcp-app-engine?rq=2

https://stackoverflow.com/questions/76068452/app-engine-custom-domain-mapping-issue-domain-not-mapping-to-app-engine-service?rq=2

https://serverfault.com/questions/849637/add-a-aaaa-and-cname-record-for-google-app-engine-project-in-google-cloud-plat (most similar to my question, but also didn't provide a valid answer)

Breakdown:

I have a subdomain (subdomain.domain.com) when adding it to App Engine it tell me to add these records:

I added the A and AAAA records, but get an error when adding the CNAME record..

I read that I cannot have an A record and a CNAME record with the same host (example subdomain.domain.com).

Does that mean that I don't have to add the CNAME record or do I need to add it differently?

I read the instructions here: https://cloud.google.com/appengine/docs/legacy/standard/python/mapping-custom-domains?csw=1#dns_update and several questions on SO, but it's still unclear unfortunately.

What the custom subdomain settings are supposed to look like

subdomain.mydomain.com -> AppEngine

the naked domain mydomain.com does not point to AppEngine

Update

Spoke to Google support and they said that the name of the CNAME record can be subdomain.subdomain.domain.com instead of subdomain.domain.com, but it doesn't seem to make sense :-/

I ended up adding all the A records and AAAA records to the subdomain and the CNAME record was added like this (as per Google support suggestion).

www.subdomain.domain.com
subdomain.subdomain.com

It doesn't seem right, but in app engine settings page it shows that everything is setup correctly.

Interestingly enough Facebook even recommends to NOT setup the CNAME at all