Is it assur to heat up matzah in the oven? During Chol HaMoed

Let’s say I want to make a pizza with Matzah and I heat up square commercial matzah for 10-15 minutes.

Would that be considered an extension of the the baking time past 18 minutes, or is it ok because the matzah has already been baked?

Thanks!

Chag Sameach