It's pointless to farm the keys. You can only open the large chest once per POE run and you get a key for completing it.
English
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Sadly a treasure isn't guaranteed. They should make it so that it is though
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Kinda true, you only get a key the first time to finish it. So future runs of the same PoE event will require you to take in your own keys
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I get the impression that's not a 100% thing.