Variation stock not handled properly

Using WooCommerce 2.6.14 I have an issue. I do use a few plugins that are affected by this issue, but I believe they are not the cause.

I have an attribute named ‘Size’ with a few values. I’ve used a bulk editor to bulk add variations to all products, one for each variation.

Now I’m trying to use a product filter plugin that can filter by Size. It doesn’t work however and I’ve found out why.

Some variations I’ve added had 0 stock. These are apparently not detected by the filtering plugin. The only solution is to find the products that contain variations without stock, add a positive stock, save the product, edit it again undo-ing the previous change in stock, save it again, and then it works.

It only seems to work when I do it manually. It does not work when I bulk add stock to the variations using the bulk editor, and then bulk save from the product list using the Actions>Edit option.

In other words, a certain plugin only works when variations have had a positive stock at least once.

Which doesn’t make sense to me.