I just discovered this for sure myself (I had suspected it for a while.) I was waiting on completing more challenges until I had Celestial armor. When battles have no gold reward, you can really see what you earn on the board. I knew something was off when I earned 193 gold every time.
I gotta say, the idea of a cap gruntles me a little bit. It greatly diminishes the value of troops that earn gold. Or souls, for that matter, though I haven’t gotten there yet. Now that I know my soul generation will be limited, I may not bother.
Has there always been a cap? Or is this new from one of the updates? I suspect it was added in an update after the developers noted it was being used to advantage by some players. Which is why there is no info box for soul/gold earners that explains their max usefulness (the way there is now info boxes to tell you the boost ratio.)
That should change. It costs 3500 or so souls to upgrade a currency-earner, and if you are doing it because you see the added advantage of soul or gold gain, you should know beforehand that the gain is negligible.
Some of my thoughts on the issue.