Data carried over stalls until new allocation is complete

I carried over 11GB of data after months of being charged out of bundle and buying more data and this time was happy that I have a carry over. Instead of starting by using the carried over bundle which expires end of December 2018, the carried over stalls until the new allocation (which expires by end of January 209) completes, then what’s the point of carrying over? I called Vodacom to enquire about it and all i’m told isn’t that, that’s how it works. I have been a contract customer with Vodacom for over 7years and don’t think i’be renewing this contract if that’s how Vodacom treats their customers!!