I've figured it out when it behaves like this. If you have a product package set on the product and if you create an order line with the contained quantity you can only decrease quantity by one whole package.
For example.
Product A has a package containing 350 units
If you make an order line with 700 units, edit and change quantity to 690 units it will create a IN move from customer.
If you decrease quantity to 350 or 0 it wont, it just updates the delivery order and pick list.
If you create an order line with 340 units, you can decrease quantity to what ever quantity, it will just update.
can anyone else reproduce this?
I have the same issue still in V15. Have you found a solution for this?
When you add a quantity that equals a package quantity, it will set the package on the sale order line, even if you decrease the quantity the package is still there (even if its not a whole package). It doesnt work to decrease the quantity on the line if the old packaging is still set (if you dont decrease a full package).
What we did as a quick fix was to release the packaging from the orderline everytime we change quantity.