Sorting variable products by price not right

Hello!

The products in my shop have multiple variations with different prices. On the category page the products should be sorted from low to high price. This is not working – they are now ordered randomly. Single products are sorted correctly and if I sort by name there is no problem. The problem is with variations and ordering them by price. If I update a product with variations the product moves in order but it is still wrong.

I have updated everything and tried with the Twenty Seventeen theme. I have tried deactivating all plugins. I have emptied WC-transients. But still the same result.

Please help!