As I said earlier - if you want to farm low tier bronze mat from event ("emerald feathers of light" in this event) - you must run standart (20AP) fights.
Because that low tier mat share the same chests with high tier mat ("hardened beak" in this event) increased chaces of drops for high tier means in-fact decreased chances for the lower one. And even if with new rigged drop you will have the same amount of lower items - you will have x1.5 more runs per AP.
PS: It is me so unlucky or the new Exper SP quest is bugged because I got only small number of bronze chests (no gold or even silver ones) from two runs?