I think that one of the timers is messed up, causing it to despawn early. Messing with the timer is difficult, so they made the easy "fix" and nerfed weapons
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The "timer" should start/reset when it is picked up by the Guardian.
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That would fix the problem, but if you killed one swordbearer, and waited until the next one spawned you might be able to use two swords against Crota at once.