Wrong available in stock quantity (decimal, fractional) calculation when editing order

[Laimonas Mituzas]

I have purchased a module "Product Properties Extension" which allows to sell products in decimal / fractional units. After testing closely, I have found calculation issue when changing ordered product quantity. Where I can find code which process this?

The full test:

New product created, quantity 32.5
https://prnt.sc/jdHaPqADaqmG
Made an Order 7.5 Left 25
https://prnt.sc/UGwkqOgbVZU_
Order edition, changing form 7.5 to 6.5
https://prnt.sc/_sblXBa-mZbl
then Changed to 8 Available 24.5
https://prnt.sc/naGfFAN4HepF
while changing quantity value, it is fine
Until I push save
https://prnt.sc/Jqaaa-Irfn8f
it shows 18. should be 24.5
https://prnt.sc/qsSKRoIrmbyD
In listing the same
https://prnt.sc/ae74jOg0S_6h
Changed order status to Shipped, quantity still the same
https://prnt.sc/zjEH9di4j1-6
in listing nothing changed
https://prnt.sc/lULKBNe1zZqX

Thank you for any tips.

PS v.1.7.8.6, php 7.4

Edited June 20, 2022 by Laimonas Mituzas
additional info (see edit history)