Came across a bug in Legend Nightfall where, if you're running solo and opening the fake chest if you move away from it before the trap is sprung, it won't register objective progression and won't complete the strike, even though the boss does spawn and drops a chest when you kill it.
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This also happened to me. i had a friend join and was able to hit the chest for loot after i had already beaten the strike and then it gave me my loot. definitely a bug tho