Variable products go out of stock on their own

Some of our variable products can’t be purchased. The variables have stock in them, but for some reason it’s “Out of stock” on the product frontend page.

When the product is saved again, without changing anything the problem is solved and the product is again available to be purchased.

For example at the moment this product has that problem:
https://www.maammekauppa.fi/tuote/ehta-by-dream-circus-amalia-mekko-baroque-flowers-mauve-chalk/

When I check the stock from edit product page, each size has stock, but the product is not available to ne purchased on the front end.

This happens randomly to different variable products on our webshop.

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