“The Mishna Berura says its a chiyuv.”
No he doesn’t. In ‘?”? ? he writes: ??? ????? ???? ???? ??? ?????? ?????? ?? ???? ???? ??? ????? ????? ???? ????? ? and in ?”? ?? he explains the reasoning behind Tosafos’s position which was quoted by the Shulchan Aruch, and he notes that several acharonim are machmir. In ‘?”? ? he says that even for those who are always careful about mayim acharonim it is ???? ???? ?? ??.