If you already have NS Flex and then change your subscription, the monthly invoice date doesn't change. This means that if you change to another subscription halfway during the month, then you will first get an additional invoice for the subscription costs for the remainder of that month, and after that you will get invoices for entire months on the usual invoice date.

This means that in your case, there was little over one third of the month left until the next invoice date, so you got an intermediate invoice for the Weekend Vrij subscription costs until that date, and later you got a regular invoice for a full month Weekend Vrij subscription costs. The detailed invoices you can find in Mijn NS > My payments clearly state for what period you are paying for.

That the latest invoice is for 30.68 euros and not just for 26.21 euros, as the Weekend Vrij subscription now temporarily costs, could be because the invoice also contains some travel costs incurred. In any case, the detailed invoices in Mijn NS will show exactly what you have to pay for.