Advanced stock management bug

[Kanava.to]

Hi,

we have a following problem (prestashop 1.5.3, advanced stock management is on):

 

1: Starting stockquantity for a product is -2

2: we make a supply order for 20 --> stockquantity is updated to 18 to -> Products details -> quantities page ...

3: An order for 1 product is made

4: Stockquantity jumps to -4 ?!

 

What could be the reason and solution to this problem?

 

Note:  We changed product setting to ”When out of stock” -> ”Allow orders”. The setting was changed after making the supply order. (between step 2 and 3, otherwise the product wouldn´t be available for buying.)