Hi,
Firstly:
When you set/change the quantity of a variant via the interface, the quantity of the main product is not changed. You need to do that manually.
When a variant is bought however, both variant and main product's quantity should be updated (that's what we have on our end with our tests).
We would need more information on how you configured a product with such issue and that you provide a link to the product page so that we could have a first look at the problem if that's really the case (please make sure that you can reproduce the problem first).
Secondly:
There is an option in HikaShop to specify when the product quantity should be changed (at the order creation, at the order confirmation). So depending this option you could think that the product quantity does not change but it will change when the order will be in the right status.
Regards,