Hi Moss
Thanks for your post.
ATUM does not make modifications in the front end in case you only have ATUM Free installed. So, we think that it has to be something you have done with the product.
According to what you tell us about what happens to it, we think that probably you have marked the product variable as "manage stock" and the stock as "0".
I'll leave you here the documentation that might be useful.
Best regards,
Iryna