There is a wrong collection source id store for a DM. I open the DM page in edit mode, open the collection section, checkmark the wrong entry for deletion and add another entry with the same collection name and the new id. As a result of the edit, the DM is not member of the collection any more. This behaviour is counterintuitive, and different from syntrans, where you can delete and add syntrans's for the same language within one edit process.