I have reviewed the delete/insert/update triggers for each sync database table. I understand these triggers add a corresponding row to the <table>_tracking table.
Most importantly we are using the last_change_datetime field to filter records as part of an import to an on_premise database from a Windows Azure database.
Why would so many _tracking table rows have all NULLs for the following fields?
update_scope_local_id scope_update_peer_key scope_update_peer_timestamp, create_scope_local_id, scope_create_peer_key, scope_create_peer_timestamp,
last_change_datetime