By design, WooCommerce doesn’t show any crossed-out regular price in the price range of a variable product when one of its variations is on sale. According to their documentation, this is to avoid long strings and prevent any confusion about the pricing.

WooCommerce Variation Prices follows the same approach for every option except for “List of all variations”. In that case, thanks to the list design, showing the crossed-out regular price doesn’t create any confusion and, on the contrary, becomes very informative, showing which ones, among all the variations of the product, are currently on sale.