Hi,
No, related products are not taken into account.
However, if you have "bundled products" then yes, the stock of the bundled products is also taken into account since the system will update the stock of the bundled products when the main product is purchased.
So in that case, it's not a bug.
Can you check if you have bundled products in your product ?
And if it's not coming from that, we'll need a link to a product with the issue and a backend access so that we can check on the situation.
You can provide that along with a link to this thread via our contact form:
www.hikashop.com/support/contact-us.html