You would report the distribution as of 2023, the year you requested it,
Depends what year the 1099R says. Did you get a 1099R yet for it? You may get it next January for 2024.
I’m in a nearly identical situation. I requested a payout in late December, my Merrill account said the request for the payout would process on December 31 2023. I received my check sometime in the middle of January. I have NOT received a form 1099-R. I’ve read that those forms are always sent out before January 31. Does this mean I’ll get one in January 2025 before I file my 2024 taxes?
You should be able to log int to your Merrill account and see your Form 1099-R, if there is one. You can also contact Merrill Lynch and enquire about it. If they have not reported as a 2023 distribution, and you want it reported as a 2023 distribution you should be able to have them correct it since you requested in 2023 and were told that it would be processed as of the end of the year.
There could be delay in the processing and mailing of the Form 1099-R due to the late request. Moreover, if it was mailed on the 31st it wouldn't be unusual for there to be a delay with the mail.