which state are they a resident of?

I don't think it makes a difference which state, no one sends a 1040-V to KC unless they are also sending a paper return with it.  I get the impression that this is a return being filed electronically but a payment being made 20th-Century style.  

Figured Id run a quick check to see if it happens to me as well, but Im not futzing around trying to guess the state involved.  Im out.

client is in NY

we are efiling the amended return but they wanted to mail in a paper check for the payment 

we know it has to go to Louisville 

I gave her the mailing envelope with the correct address 

must be a summer bug. . . . 🤣

It doesn't really matter.... they'll take the $$ at either address....  Accepting money is probably the only thing the IRS does well.