GCP DNS confusion with Google Domains
Solution 1:
Google Cloud DNS and Google Domains nameservers are not the same, see documentation: https://cloud.google.com/dns/quickstart#update_your_domain_name_servers
Even if your domain name is registered with Google Domains, you still need to update the name servers — although they look very similar, the name servers used by Google Domains are not the same as those used by Cloud DNS for your managed zone.