Once the product details page is loaded, the product's price is not affected by the prices of the custom fields (the first options which are selected by default).
The price is updated normally after selecting a custom field.
VM. 2.9.9.5
The same in VM3 stable
This is intended, else the price is always recalculated, and that in a category browse view for 50 products with ajax is quite bad.
and actually it can be avoided if you give the product the correct price of the first variant.