my configurations is same as you described but it is not working when a partial sales occurs, back order is in waiting state until we create a purchase order and buy products

The reordering rules runs on schedule (or you can run it manually) and the purchase order qty will be calculated depend on the maximum qty and multiple in ordering rules (Not depend on the sales remaining qty)

Yes Waleed it works same. But if you defined min and max 0.00 in reordering rules and if your product configured route 'Buy' system creates purchase order for the 5 qty. (of course on scheduler run.)

@waleed Check here : https://youtu.be/_ciHvkyZBxw

You are right