The behavior you are describing is called SSO. It's normally up to the client to say if the `acr` in the id_token is not acceptable.
See this issue if you want to hack stepped up authn on the IDP.
https://support.gluu.org/authentication/11863/how-to-stepped-up-authentication-for-application/
And I'm closing this issue because the support entitlements of the customer don't cover this issue, and Sennovate is not a partner in good standing because they've never sold a single Gluu customer.