Do cinnamon, mint and saffron turn bitter?
cinnamon, mint and saffron are bitter.

You probably know that "flavor" is roughly 90% smell. Our tongue identifies only the basic tastes, such as sweet, sour etc. When you "taste" mint, your tongue identifies a sour-bitter mixture, and it's your nose that absorbs the citrusy-herbishi-whatevery aromas that we call "mint".

When you cook spices, and especially when you overcook them, the gentle volatile aroma compounds veporize away or are destroyed by the heat. And you're left with the plant material, sometimes wilting, mostly without the smell.

Now, of course the "tongue" taste of these herb and two spices is not straight-out medicinal bitter. But I guess it's not what you meant either. It's just.. a bit yakkish, on the bitter side, right?

Here's what I do to avoid it: I add gentler spices towards the end of the cooking. Say, in a slow cooking dish, I'd put a whole cinnamon in the beginning, but the fresh chopped mint will be added only when I serve it. So to your question - yes, it's heat and quantity. Put more, heat it less.