I confirmed with the HDFC Customer support.
If any purchase has happened on the credit card and the credit card bill amount is paid within 3 days after the purchase then the HDFC bank auto-refunds the amount back to your account with this statement and code.
E.g.
Credit Card Statement :
Date Details Amount
30 Oct 2025 Amazon Seller Services Mumbai In ₹1,304.00 Debited Step-1
30 Oct 2025 Credit Card Payment Net Banking ₹1,304.00 Credited Step-3
31 Oct 2025 Dual Pyt Refunded To Ac X ₹1,304.00 Debited Step-4
Savings Account Statement :
30 Oct 2025 IB BILLPAY DR-HDFC4Q-36XXXXYYYYZZZZ ₹1,304.00 Debited Step-2
31 Oct 2025 EXC PYMT DUP - 0036 XXXX YYYY ZZZZ ₹1,304.00 Credited Step-5
06 Nov 2025 IB BILLPAY DR-HDFC4Q-36XXXXYYYYZZZZ ₹1,304.00 Debited Step-6
As you can see above esp. from step 1-5, the purchase on credit card and also the credit card bill payment happened within 3 days ( in this case on same day I paid the CC bill) due to which on next day 31-Oct the payment is refunded.
Note : The last 2 columns : debited/credited and step-N have been added by me for easier explanation.
I have no auto-debit enabled for the credit card and still this happended.
So, as you can see in step 6, after speaking to customer support I have again paid the bill to test this as 3 days have passed since the purchase of 30-Oct.
I expect this time the auto refund should not happen and on 15-Nov which is credit card cycle everything should be settled properly.