In EAP 7.1.0 GA elytron will be default security solution, therefore default https should be backed by elytron ssl context rather than with deprecated legacy security realm.

Default undertow https listener use legacy ssl context in standalone-elytron.xml .

<https-listener name="https" socket-binding="https" security-realm="ApplicationRealm" enable-http2="true"/>

Once elytron becomes default security solution in eap it has to use elytron ssl context. In meantime prepare such configuration in standalone-elytron.xml

<https-listener name="https" socket-binding="https" ssl-context="elytron-ssl-context" enable-http2="true"/>