Discounts inconsistent on variations

When applying discounts to a variable product the discount is displayed properly on the product but only before any variation is selected. When the variation is selected the old and new price appear the same. Adding the variation to the cart displays it there with the correct discounted price.

So the problem is that on single product pages variations show the new and old prices the same. Not selecting any variation displays the correct new discounted price for the parent product.

I tried different themes and it always behaved like this. What could be going wrong?