Not sure if anyone else has noticed this, but I decided to go back and farm up some mission rewards I skipped in the story missions.
I’ve spent more than 20 matches on various missions where I only have 1 of 4 rewards. Every time, my only reward is 20 iso. This is not just bad rng - it seems it’s literally impossible for me to obtain the other rewards.
Obviously, this is a little frustrating. 
Just to clarify, this only applies to old nodes. New nodes give one reward, then start handing out 20’s like there’s no tomorrow.
If anyone could confirm this is also happening to them, I’d appreciate it.
I remember reading somewhere that you have a 50% chance, at getting a reward, but I can see your frustration. Not sure if there is an error. Can’t check myself, all my prologue nodes are done.
You’re just having bad luck. For any story/PvE node with less than full (but more than 0) rewards remaining, you have a50% chance of getting one of the good rewards. Fail that coin-toss, and you get 20 ISO.
It might seem like the game is broken, but it’s just a run of bad luck. If you were having a run of good luck on these rewards, it would never even occur to you to suspect a bug.