I got this bug from time to time (around 25% chance?): Despite having 110%+ map participation, my end reward chest only counts ~30% of my map participation resulting in 1 key and 15 aurillium instead of 75 aurillium and 7 keys.
My guess is that the game sometimes calculates the chest too early, because you can get 80% map participation from helping out in all 4 lanes.
1st person worldwide to reach 35,000 achievement points.