@KameofWar has already explained this multiple times in several threads. Click on his name, go through the posts and not only will you find the actual numbers but a better explaination as to what I’m about to attempt…
Of course they don’t stack, one is for ranged attacks and the other is for melee attacks. Attacks being the important word; so using the combi Stick as an example, why should you get the bonus damage from Downrange when using it in melee? Why should you get the bonus from Fearless if you’re throwing it? Same with Smart Disc.
And why would they stack to begin with? There are no RANGED MELEE attacks, that’s an oxymoron.
If you’re running Fearless and Down Range, and say for example are using the Combistick the game gives you your bonus accordingly. So you’ll get the Fearless bonus when in melee and you’ll get the Down Range bonus when you throw it. It isn’t a bug, it’s how they’re designed to work.