Okay, so the Public Event at the Sirens Watch on Titan does have a bug or something with the reward chest. Everything is fine with the whole encounter just when the heroic version is activated and the walker tank starts moving towards the area where the chest spawns. When he is killed the chest won't appear because the walkers dead corpse is laying in the way. I've encountered this many times here on Titan for this Public Event only. If you may Bungie fix this issue that'll be much appreciated! :)