You can explore dependencies between Kibana saved objects in the dedicated page in the management application. Given a dashboard it can point directly to data views or to visualizations. Filbeats dashboards pint to visualizations, so you can check for those relationships. I've just tested in 8.4.3 and they point to the only created data view filebeat-* as you mentioned.
The data view should pick up all the indices given the name pattern. If it's not picking the data from your additional index I'd suggest you start checking the visualizations and the query they perform from the Inspect tool on the top right of Kibana to see why they are not getting the results you expect. Is the schema in your new index conforming to ECS as the default filebeat indices?