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 wildfly 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"/>