Fresh from their own 3-1 away win seven days previously, the Pilgrims were beaten by the same scoreline at Freiston FC.
Will Jackson was United's goalscorer at 2-0 — but Boston conceded again to lose by a two-goal margin against Steve Welsh's side.
Boston (4-1-4-1): Jake Frestle; Will Jackson, Taylor Gray, Daniel Fisher, Nathan Stevens; Marshall Young (Finn Kirke 65); Xander Crashley, Lewis Bontoft, Kyle Tate, Jenson Bark; Harrison Holland (Mido Turki 46).