that’s if you tap on ‘Pay off extra’ in the list of selections in the ‘Manage’ tab of the Flex card. In my case it shows the total of all outstanding Flex payments rolled into one (£66) from which to pay a chosen amount off. Straightforward so far.
But if you go into the ‘Manage’ tab of the Flex card and instead tap on ‘Next Flex payment… £xx.xx’, you get the chance to pay off a fixed amount that doesn’t indicate it will come off the last payment. It shows an amount which equals the next payment due - at least in my case it does:
Complicated character this Flex is.