This could be subject to change but from what I have seen the IRS is only doing direct deposit of the stimulus payments based on the direct deposit information ...
At this current time the IRS is only using the bank info from the 2018 and 2019 returns, if no bank info is on either return then the IRS will mail the stimulus...
If you filed electronically the IRS should reject the return if your last name is not correct, you can them resubmit with the correct information after the reje...