First the IRS is not always correct and if you mailed in the return the data input operator probably missed the LSE notation on the form 1040.
Next this could be a user data entry error ... when you made the LSE you had to enter the 2020 tax information so the program could calculate how much of the SS benefits on line 6a were taxable on line 6b. So save a PDF of the return with all the worksheets to see if this was your error or the IRS. If it was the IRS send the calculation worksheet to support your position.
