There may have been a change since your code was added. I tried your flask-google-oauth2 tutorial code.

I was getting this error:
RuntimeError: Missing "jwks_uri" in metadata

I added this line to the oauth.register:
server_metadata_url='https://accounts.google.com/.well-known/openid-configuration'

Then this error occurred:
authlib.jose.errors.InvalidClaimError: invalid_claim: Invalid claim "iss"

I had to remove openid from client_kwargs to get it to work.

Did I miss anything?