Hi,
The variant quantity would override the product quantity.
But "0" on the variant is not really considered as an override value.
If the main product does not have quantity and variant have, it would work.
If the variant does not have quantity and the main product have, it would considered that there is a possibility to add the product in the cart.
That's why, when you have variants and you want to configure the stock for your products, the main product quantity should be 0.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.