If so why is this?
I cook tomatoes in mine, but only when tomatoes are just a portion of a recipe. I have never noticed a metallic flavor. I would not, however make a tomato/marinara sauce in my CI pan. It's the acid in the tomatoes that supposedly react with the iron. My pan is also quite old and well seasoned. I would avoid tomatoes if your pan is new.
You are very welcome. :0)