As described in https://feyncalc.github.io/FeynCalcBook/ref/SetMandelstam.html, the arguments are SetMandelstam[s, t, u, p1, p2, p3, p4, m1, m2, m3, m4]; these masses should not have been squared. These are the only occurrences of /SetMandelstam.*\^/ in the repo.
As described in https://feyncalc.github.io/FeynCalcBook/ref/SetMandelstam.html, the arguments are SetMandelstam[s, t, u, p1, p2, p3, p4, m1, m2, m3, m4]; these masses should not have been squared. These are the only occurrences of /SetMandelstam.*\^/ in the repo.