Filter by attribute – variable products not showing

Same issue as https://www.ads-software.com/support/topic/product-filter-by-attributes-disappeared-for-variable-products/

Updated WooCommerce to latest version and hoped it would be fixed but the issue remains.

I have a “size” attribute and a sidebar with filtering by size.

On the main shop page, it shows:

Filter by size

3 months (1)
50/56 (1)
56 (2)
62/68 (4)
98 (1)

Yet when I click the parent category (baby clothes) which applies to all of those items, then the sidebar widget shows:

Filter by size

3 months (1)
56 (2)
62/68 (2)

The simple products are shown, but the variable products are not.

Please help.

The page I need help with: [log in to see the link]