If you look closely at the swordbearer decloaks just as they are approaching Crota. Thus, Crota was able to spot them and kill them with his sword.
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Looked over it again and I find it worried because it takes 3 gally shots to take him down on hard and I shot 2 and there was at least 1 more so he should have went down but my friend said it was lag that causes all the glitches