See long shaarei teshuva siman tod samech. In a nutshell, they used to make matzo meal from thick Matzoh which had a real chance of not baking thru as the producers were in a rush. However ours is made of same thin matzos that we eat with the same neglible risk of chametz. So its a minhag based on historical risk. Which should not impede on simchas yom tov. However those who are stribgent should not be chastised.
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