Product quantity when 0 turns to unlimited

Hi,

Yes. First, I would recommend setting up this plugin:
www.hikashop.com/marketplace/product/161-action-log.html
So that your website can log what's going on. That should give you and us a better understanding of what is happening once the problem is reproduced.
Then, please provide a backend access with information on the product you identified with the issue so that we can check its settings, the orders related to it, the massactions potentially related to it, etc.
Also, please tell us if you have custom plugins.

Hi,

You have configured this mass action:
i.imgur.com/Xw2L0ff.png
Every hour, it automatically changes products with a stock of 0 to unlimited (-1).
So it's this mass action which does this I think.