The amount for MF purchases is debited from your trading account and not from your bank account.
Note that the amount for all Coin mutual fund orders placed before 1.30pm will be debited at 3.30pm on the same day. If the required funds are unavailable at 3.30pm, then it’ll be carried forward either until the amount is available in your trading account or you cancel the orders.
You can also fix a Standing instruction from your bank account to your trading account on the day before your SIP due date as well to ensure your SIPs always execute. Check the below link for more: