Variations in-stock on backend not showing up on the frontend

Hi there

I found a bug in woocommerce. We have an ecommerce site with the below attributes. We use all import to import the products every season. On the jeans specifically we have a waist and length attribute but not all variations would be made out of the attributes as there is no product in that specific size etc. The problem is that even if that product is in-stock on the backend with the correct settings it is not showing up on the front-end and. 

I created numerous test products and saw that if I create variations from all attributes the products shows fine on the front-end if it's in-stock but in our case when only some attributes are being used for the variations there is an issue. I have explained below what we currently have on the site. 

This is a massive issue as products in-stock are showing out of stock. Please can you assist urgently.

We have the following attributes
Waist:? 30, 31, 32, 33, 34, 36
Length:? 30, 32

But only the variations below are in the import sheet and sold as actual products
Waist 30 - Length 30
Waist 31 - Length 30
Waist 32 - Length 32
Waist 33 - Length 32
Waist 34 - Length 32
Waist 36 - Length 32

We don't have the variations below because we dont have these actual products in these sizes
Waist 32 - Length 30
Waist 33 - Length 30
Waist 34 - Length 30
Waist 36 - Length 30
Waist 30 - Length 32
Waist 31 - Length 32

I would have to log a ticket with woocommerce and describe the issue to them and see if there is a fix , otherwise we would have to create a new import sheet on all products with the waist & length and just set the stock quantity on 0.

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