Sales order quantity changed incorrectly

[kivo]

I made a sales order of qty 100each of an item. Then dispatch 10each for first invoice and the Action for Balance is set to "Automatically put balance on back order". After process this invoice. I saw the sales order qty still keep 100each and invoice 10each. That's correct. Then I dispatch another 20each and now the Action for Balance is set to "Cancell my qty not deliver". After process this invoice. I found that the sales order qty changed to 20each only. And the order delivery difference report shows that the difference qty is 80each.

I think the software put my last dispatch qty back to the sales order qty and generate this error.

I am using WebERP 4.10.1