OK ... I may have an idea as to why this happened ... if you had unemployment on your return AND you did not exclude it THEN when the IRS corrected the return to give you the unemployment exclusion your income is now low enough to get the credit that was NOT included on the original return. SO to get the credit the IRS requires you to now fill in the EIC form so it can be added to your return when they FINALLY process it. This is NOT anyone's error if you filed before the exclusion was signed into law .... the IRS is making this stuff up as they go which is why the form 15111 was not known about until they put it into service recently(note the date on the form March of 2021). Everyone should be happy the IRS is catching this and not forcing folks to amend their returns since many folks would never know they were able to amend for the credit ... they are being proactive for a change.
PS ... if you take a look at the 15111 and the EIC forms they provide the same info and the ability to sign the form so you can file it by itself as an addendum to the return ... no amendment needed.
https://www.irs.gov/pub/irs-pdf/f1040sei.pdf
https://www.irs.gov/pub/irs-pdf/f15111.pdf
https://www.irs.gov/individuals/understanding-your-cp09-notice
https://www.irs.gov/credits-deductions/individuals/earned-income-tax-credit-eitc
