When you say most people say paysach, you mean only when talking about the yomtov paysach. When reading the word in a siddur, machzor or chumash etc then almost all ashkenaz non chassidic readers will say pesach eg kol shelo omru shelosha devarim eilu bapesach… V’eilu heim… Pesach, matzo, maror.
Has anyone ever seen the word pesach with a tzeirei under the letter Pay instead of the usual segol? This would explain both the litvish paysach and the chassidic pie-sach.