FORM fallback mechanism

With for instance SPNEGO it is possible to set FORM as a fallback mechanism. In order to let special users log in using a FORM instead of KEYCLOAK-SAML we intended to apply this kind of behaviour to our SAML application too. However, if I add the Keycloak adapter (6.0.1 on Wildfly 13 with Elytron), it overwrites the mechanism defined in the web.xml of the war. This means that when I then try to log in using a form, the mechanism isn’t recognized. Is this functionality lacking in the Keycloak adapter or did I misconfigure something?