Renewed contract in good time and first bill doesn't match the new monthly service charge

I had a contract which ran from 17th August 2022 to 17th February 2024. I then called your support team on 16/01/2024 to renegotiate a new contract as the monthly service charge fee was going up. However, the first payment doesn't match the new negotiated monthly payment price and has got an additional 50% added on. I called support about this and they said it was to do with the billing period and the monthly service charge for the end of your current/previous contract. This doesn't make sense to me and feels very deceptive. I've got two Virgin Media accounts with you and one of them is 12 years old now - I can't recall a time when this has happened before. Can you assist and explain?

Looks like I'm not the only one as well https://community.virginmedia.com/t5/Managing-Your-Account-Cable/new-renewed-contract-first-bill-does-not-match-contract/td-p/5468395