I have noticed that when I use either of the three (cinnamon-mint-saffron) spices/herbs in savory dishes they tend to turn them bitter. With the saffron I know that you only need a pinch but even that pinch seems to be turning the food bitter (stovetop cooking). Has the issue got to do with the heat or the quantities or both?
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.
This might be worsened if the spices are dated: The always-present bitter compounds last longer than the aromatic compounds that make the bitterness worth dealing with. So you need to use more, making the dish bitter.
Also, you could be undersalting (reasonably bitter, alkaline or hot-spicy dishes sometimes taste oversalted to me when not paying close attention to what the exact taste is that seems oversalted... salt in excess is bitter too!) ... when the dish is actually undersalted or underacidulated! Separate a small portion out and add salt/vinegar and taste if you are not sure). There is a reason heavy curries (lots of bitter spices in there) are on the salty side and also need balancing by some sugar and acid. (VBMC Ep. 7 is right on the money ;)
Spices do mostly not mess with taste (except bitter, where it is nowadays controversial whether bitter is only taste dimension as has been assumed for a long time...), they mess with aroma - so if the basic 6-tastes seasoning is far off the mark a spicy dish will taste "dissonant" easily.
BTW, "cinnamon" can mean two different spices that are sold as "cinnamon" -"ceylon cinnamon" and "cassia bark".