Minimum quantity change impossible

[tattum]

Hi,
I use Prestashop 1.6.0.9 and a paid theme.

 

I have created a list of products.
At first, none of them had combinations and I set up order minimal quantities to 10 or 20 (Catalog>Products>Quantity>Availability setting => change minimal quantity)
 

Then, I updated some products by adding combinations. I also wanted to change the minimal quantity, but the "minimal quantity" field has just disappeared, so I can't proceed any change.

Those who don't have combinations don't have this issue.

 

Any idea anyone?