Hongkong & Shanghai Banking Corporation [Hsbc] — terms & conditions for preclosure of balance transfer not clear

I have availed balance transfer from HSBC to clear another credit card dues.
After around 5 months, I cleared all the dues of HSBC on 02 Mar 2018. From then on, I have not used the card for any transaction. The due I cleared is exactly the total loan outstanding as per HSBC statement.

In the statement of September month, I received a minimal due of Rs.150.62/-. In the statement of October month, the due is Rs.5857.23/-. When I contacted HSBC customer care, I was told that for preclosure of Balance transfer, I need to contact their PhoneBanking and notify them, else the loan is never closed even if the amount is paid.

I cross-verified, if this clause is mentioned in the automated voice-based Terms & Conditions, which is played for every customer when the Balance Transfer loan is availed. This clause is nowhere mentioned.

I see this as clear motivation of hiding terms to misuse!!

Customer Care executive asks me a question, "How would the bank know that the Balance Transfer Loan is preclosed, unless it is informed of?"

My question is, "How would a customer know that he need to call HSBC Phonebanking to preclose a loan, if the same is not mentioned in T&C. That too, even after paying all the dues?"