Vistara Airlines — Ticket cancellation amount not refunded

On 23 July 2022, I booked a "flexible" Vistara flight ticket for Delhi to Kochi for 12 August 2022 by paying extra amount for a full refund ticket. I cancelled my ticket on 7 August 2022, expecting a full refund of the amount. However, as of 27 October 2022 the amount has still not been credited to my bank account. The Airlines has not provided a satisfactory tracking number using which the transaction can be tracked by the bank. Even after 3 months of follow up the Airlines has only provided a vague notepad file as proof of refund, which is not a valid invoice.