While working on an add-on for sale.order.line I noticed that the sale_line_id on the stock.move entries only has sale_line_id populated for the delivery stock move.

Is this the expected behavior? Or should sale_line_id always be populated on every stock move?

Without it being populated, a 3-step delivery operation does not trigger any depends for "move_ids" on the first operation being completed. It'll only see the delivery move changing from awaiting operation to waiting and then ready and done.