Both are in some sense correct. When changing from Standard to Flex, what you are actually doing is getting a new ticket and paying for it with the value of the Standard ticket plus some other method of payment.

So if the Flex ticket is refunded, it will refund to the method of payment: the difference between Flex and Standard in cash equivalent and the value of the Standard ticket as a voucher (likely with the same expiration date as when you'd cancel the Standard ticket, but some airlines implement the voucher with a reset expiration date).