Keycloak/OIDC : retrieve user groups attributes

I've extracted a user's groups information from the OIDC endpoint of Keycloak, but they don't come with the group ATTRIBUTES I defined (see Attributes tab into the group form, near Settings). Is there a claim to add to my request?

I'm using a RESTeasy client to reach Keycloak's admin API (had much better results than using the provided admin client, yet):

@Path("/admin/realms/{realm}")
public interface KeycloakAdminService {
    @GET
    @Path("/users/{id}/groups")
    @Consumes(MediaType.APPLICATION_JSON)
    @Produces(MediaType.APPLICATION_JSON)
    List<GroupRepresentation> getUserGroups(@PathParam("realm") String realm, @PathParam("id") String userId,
                                            @HeaderParam(AUTHORIZATION) String accessToken);
    //DEBUG the access token must always be prefixed by "Bearer "
}

So I can fetch a user's groups:

private void fetchUserGroups(UserInfoOIDC infos, String userId) {
    log.info("Fetching user groups from {}...", getRealm());
    try {
        KeycloakAdminService proxy = kcTarget.proxy(KeycloakAdminService.class);
        AccessTokenResponse response = authzClient.obtainAccessToken(getAdminUsername(), getAdminPassword());
        List<GroupRepresentation> groups = proxy.getUserGroups(getRealm(), userId,
                "Bearer " + response.getToken());
        infos.importUserGroups(groups); //DEBUG here we go!
    } catch (WebApplicationException e) {
        log.error("User groups failure on {}: {}", getRealm(), e.getMessage());
    }
}

But when it comes to data exploration, it turns out that no attributes are provided into the GroupRepresentation#getAttributes structure.

I've read that claims can be added to user info requests. Does it work on the admin API? How can I achieve that result with RESTeasy templates? Thx

This is how I could eventually map group attributes (inherited as user attributes, as suspected before) into user informations, into the "other claims" section :