Kvod Rabenu Shlit’a. Where is the source in Halacha of not placing matza during Pesacha in liquids or eating gebrochts?
A. Likely one of the first reference to Poskim that mention the above Minhag is the Olas Shabbos (453:3) who discusses a Halacha of the Bach that Matzos should be baked only prior to Pesach and not during Pesach. The reason he mentions is that Chometz only becomes batel or annulled before Pesach and not during the Yom Tov. The Olas Shabbos adds, “And one who is concerned for this stringency should not cook a baked Matzoh on Pesach either, because how and why is it different than baking?’
In other words, the Olas Shabbos is concerned that the Chametz that was mixed into the Matzoh but became nullified before Pesach, may be now re-awakened on Pesach itself.
The Mogen Avrohom (458:1) quotes the Olas Shabbos and after questioning it, differentiates between the Bach’s and the the Olas Shabbos cases.
Another source for the concern for Gebrokts is in the response of Rabbi Shniur Zalman of Liadi, the Baal HaTanya. He raises concern that perhaps the Matzoh was not kneaded sufficiently, and some unbaked flour may remain on the Matzoh. The soaking will thus bring about chometzing or fermenting process.
Rabbi A. Bartfeld as revised by Horav Yaakov Hirschman, Horav Dovid Pam, Horav Aharon Miller, Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a.