Google Cloud DNS - Subdomains wildcard not working

As @John Hanley mentioned, Google Managed SSL Certificate does not support wildcard (*.example.com).

I've just created a new certificate with Letsencrypt and wildcard seems working.

If you are using a Google Managed SSL Certificates then wild cards will not be supported. You can use wildcards to map subdomains at any level, starting at third-level subdomains.

Kindly refer to the below link for:

Mapping custom Domains : https://cloud.google.com/appengine/docs/standard/python/mapping-custom-domains#wildcards

Securing Custom Domains with SSL : https://cloud.google.com/appengine/docs/flexible/python/securing-custom-domains-with-ssl