The Shulchan Aruch (462:1) writes: Fruit juice without water, can’t cause something to become chometz, and it is permissible on Pesach to eat matzah kneaded with fruit juice, even if it has been sitting all day. However, one can’t fulfil the mitzvah of matzah with it, as it is matzah ashira and the pasuk requires that one use lechem oni.
However, the Rema (462:4) writes: We are stringent about this, except in pressing circumstances such as for someone who is ill, or elderly.
The Mishnah Berurah (s.k. 15) writes: Lechatchilah we are worried for the poskim who hold that fruit juice itself can cause something to become chometz. We are also worried that a little bit of water may have got mixed into the mixture, which according to everyone can cause things to become chometz. Therefore, some say, that this year it is permissible to eat matzah ashirah on Shabbos until the chometz eating deadline, and if one is kovea seudah [fixes a meal] on it, he may make hamotzi and birchas hamazon (see Igros Moshe, Orach Chaim 1:155). However, some say one shouldn’t eat matzah ashirah even on Friday night (see Shaar HaTziyun 444:1 in the name of the Gra, who learns that according to the Rambam matzah ashirah is included in the prohibition of eating matzah on erev Pesach. See at length the sefer Erev Pesach Shechal BeShabbos, Perek 8, he’orah 3. And Shevet HaLevi in Koivetz MiBeis HaLevi, Koivetz 5, pg. 63 that the minhag of Ashkenazim is not to eat matzah ashirah, and one should use regular challah.)
