I would assume it's because the status values in RO had changed (reduced) from v16 to v17.

v16 had:

('draft', 'Quotation'),
('confirmed', 'Confirmed'),
('ready', 'Ready to Repair'),
('under_repair', 'Under Repair'),
('2binvoiced', 'To be Invoiced'),
('done', 'Repaired'),
('cancel', 'Cancelled')

v17 have:

('draft', 'New'),
('confirmed', 'Confirmed'),
('under_repair', 'Under Repair'),
('done', 'Repaired'),
('cancel', 'Cancelled')

so you can se that v17 is missing two: 

('ready', 'Ready to Repair'),

('2binvoiced', 'To be Invoiced'),

maybe your solution checks if the status changed from "under_repair" to "2binvoiced"?