Reported here (in German)

Kartographer only supports OSM relations of the types multipolygon, route, and boundary. Many public transportation routes, however, have the type route_master (documentation), which combines the individual route relations (e.g. both directions) (example).

The current workaround is to add the QID to each individual route relation, but this is neither simple nor intuitive for wiki editors, and might not be accepted by the OSM community much longer. In consequence, maps for public transportation can hardly be used.

See also: T156433