An extent is marked as invalid if DELETE, INSERT, UPDATE touches this extent. The extent has to be reread by SELECT to become valid again. If one bulk inserts in the invalid extent then min/max values aren't updated in the EM.
One should read the data out after DML operations and before you bulk inserting data to make extents touched by DML valid again. To reduce number of extents to read one could set an impossible filter to eliminate all valid extents.