Hi Wayne, thanks for the information, we will look into the portal notifications. 
- If over-delivery is ticked and you deliver more than you have in stock, the system logically does not know what the extra items are worth
- If the customer returns those items, then again the system has no idea of their worth
- However, the system should at least pay attention to items if they are not in negative stock.
This is why the maintenance routine worked when you un-ticked that setting. 
I hope that this explanation of the system logic helps :) 
We will do further investigation on possibly making enhancements to this in the future and will let you know of any improvements 
Best, 
Omar