Strange calculation then manually add item in order

[wer_ru]

Hi!

Example.

I have an order. I want to add new item after conversation with a client.

I seek and find "item1  10 in stock"

I added 2 pcs

And I see in order

"item1  2pcs   6 in stock", not 8 as should be

Could you help me to understand where I should start to find the problem!

I tried

- empty cache

- change php version

PS 1.6.1.17

 

 

 

Edited December 9, 2017 by wer_ru (see edit history)