Assertion is not signed with custom domain

Hi Auth0 community,

I’m integrating a K15t app with SAML integration to Auth0 as identity broker to either Azure or AWS Cognito.
See Set Up SAML Single Sign-On.

The integration works perfectly without using a custom domain, after downloading metadata from the Auth0 app (type is Regular Web Applications) addon SAML2 Web App.

Now I would like to use a custom domain instead in the login screen.
I created a custom domain, I updated the callback URLs in Azure and Cognito (both integrated with Auth0 via a social connection), I recreated (remove+setup) the authentication in Scroll Viewport (where to configure the K15t site in Atlassian Cloud) to apply the metadata of the custom domain which I downloaded from https:///samlp/metadata/.

However, when I login to the K15t site using the Auth0 custom domain, I get this SAML Authentication Error:
Error message: Assertion must be explicitly signed.

What did I missed ?

Thank you in advance for your help,
Best regards,

For more clarification, from the documentation Configure Features to Use Custom Domains, I did Configure Social identity providers, and Configure SAML applications.

The Auth0 app addon SAML2 Web App is configured with this Settings code:
“audience”: “”,
“signResponse”: true
Does that mean the signResponse is ignored when using a custom domain?