Mapping subdomain to GCP app engine with wildcard

I'd like to map my_domain_name [URL=https://www.sipotek.net]SIPOTEK[/URL] and any subdomain of my_domain_name name to the same App Engine ("backend") in Google Cloud platform.

I follow the step of this documentation. https://cloud.google.com/appengine/docs/standard/python3/mapping-custom-domains

I deployed this dispatch.yaml

> - url: "my_domain_name.com/*"
> service: backend
> - url: "*.my_domain_name.com/*"
> service: backend

Settings/Custom domains looks like this : Custom domain name

.my_domain_name.com/* (SSL Security : Google-managed, auto-renewing) *.my_domain_name.com/* (SSL Security : None)

And I've filled appropriate CNAME, A, and AAAA records on my registrar (OVH).

It works on my_domain_name.com, but on xxxxxx.my_domain_name.com, I got a security "Failure in secure connexion error". Is it related to SSL security ? How can I fix that ?