Thorntail MP JWT / Undertow: required authentication

I'm trying to set up a JAX-RS-service in thorntail with JWT authentication. Everything works fine (I can inject Principal and user is correctly set), except that in case of a failed authentication, answer is still sent without any 401-HTTP-Header. What I've done is:

• Added @LoginConfig(authMethod = "MP-JWT", realmName = "my-domain") to my Application-Class

• Configured the security-domain

  security:
    security-domains:
      my-domain:
        jaspi-authentication:
          login-module-stacks:
            roles-token-stack:
              login-modules:
                jwt-jaspi-login-module:
                code: org.wildfly.swarm.microprofile.jwtauth.deployment.auth.jaas.JWTLoginModule
                flag: required
        auth-modules:
          http:
            code: org.wildfly.extension.undertow.security.jaspi.modules.HTTPSchemeServerAuthModule
            module: org.wildfly.extension.undertow
            flag: required
            login-module-stack-ref: roles-token-stack
  

• Configured JWT-specific things (seem to work, so I'm skipping this here)

What else do I need to do in order for this to work properly? Do I need to add any annotations to my Endpoint? As I said, I want to return a 401 in case of a failed authentication.

What I've found out so far: JASPICAuthenticationMechanism.isMandatory needs to return true in order for this to work. If this is the case JWTAuthMechanism.sendChallenge is triggered after a failure of JWTAuthMechanism.authenticate and so a 401 is sent to the client. But i have no idea, in which cases isMandatory returns true.

Thanks for any help in this case!

Solution (thanks to Ladicek, see comments below):

If you want to use MP JWT, don't start it with Swarm and don't forget to set flag useUberJar if starting it with thorntail:run.