Hi lavieenrobe,
Thank you very much for using ATUM and for opening this post.
We are very sorry you are having this issue.
However, we can guarantee that this is not caused by ATUM. The plugin does not change any stock at its core and only shows info pulled out of WooCommerce.
Knowing the WooCoomerce, we can think of only one way the stock would come to -1 level. Our team has tried a scenario where “back orders” are allowed at the product level. If your stock level shows 1 unit and the buyer purchases 2 products your stock would automatically show -1. Please, check your product settings within the inventory tab.
We hope this helps.
Best Of Luck,
Pavel
Stock Management Labs?