I started Social Security in 2018, changed my mind, and repaid the benefits. Because the repayment hadn't yet been reported to the IRS, I included the benefit when I filed the 1040. I just received a corrected 1099, showing '0' benefits received. Using TurboTax to prepare the 1040X, when I delete the amount that was reported, TurboTax indicates the "net change" as a lower amount. I don't know where this number is coming from.
UPDATE: According to a TuroTax CPA, the 15% reduction I'm seeing is because 15% of the benefit wasn't taxed.