The existence of bend points in the formula means it’s non linear. https://www.ssa.gov/oact/cola/bendpoints.html

PIA formula For an individual who first becomes eligible for old-age insurance benefits or disability insurance benefits in 2019, or who dies in 2019 before becoming eligible for benefits, his/her PIA will be the sum of: (a) 90 percent of the first $926 of his/her average indexed monthly earnings, plus (b) 32 percent of his/her average indexed monthly earnings over $926 and through $5,583, plus (c) 15 percent of his/her average indexed monthly earnings over $5,583.

Note the c is capped 11,075 US$ per month as income above that is not taxed.