This is the query that we run in the USED BY tab:
with Objs as
(Select object_id
from (select object_id, referenced_object_id
from public_dependency
where referenced_object_id <> object_id) pd
connect by nocycle prior object_id = referenced_object_id
start with referenced_object_id = :ObjID)
Select o.owner, o.object_type, o.object_name, o.object_id, o.status
from sys.DBA_OBJECTS o, Objs
where o.Object_id = Objs.Object_id;
So, Oracle must be showing your Materialized views as invalid in DBA_OBJECTS. This can happen is if you've done any DDL on any of the objects referenced in the MView's query.