Social Security is on 1040 line 6a and any taxable amount on 6b. You get 0 on 6b if you didn't answer the Foreign question right. Or if the amount on 6b is too low and you are married you answered it wrong for one spouse. When did you file? If it was early there may have been an update later to fix that.
Or the IRS changed some other income on your return. Like the taxable amount of a 1099R you got or you missed entering some income. Like if a 1099R got entered on the wrong line 1040 line 4 or 5, the IRS would miss it and think you didn’t report it. By increasing your other income it made more of your SS taxable.
