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Doesn't sound right, but it's impossible to answer without knowing what type of mortar you used, how wet the mortar was, how well it stuck to the bricks, how wide the joints are, etc., etc. If there was a fair bit of mortar standing proud of the joints, that would naturally fall off when the oven is fired the first few times.
How about some specifics?
"Made are tools, and born are hands"--William Blake, 1757-1827