It’s not clear how the gong/chest bonus loot mechanic is supposed to work.
We rang the gong on boss 1, but didn’t loot the box.
We then disabled the pylons on 2 and 3 and reached the checkpoint.
When we rang the gong after boss 3 and looted the chest, we could backtrack and loot the chest at boss 1, then ring the gong and loot the chest at boss 2.
I don’t know whether we “cashed in” early and cancelled our descent into madness or not. I assume not, because we still have the debuffs, but it seems weird if we’re intended to go back and loot 3 different boxes, because the bluepost implies that the loot from the previous bosses is stored up and added to the box at boss 3.