I just did 37 legendary lost sectors and got 2 drops. That's 5.4%. Don't believe me? Check my play log. I played for nearly six hours for 2 drops, and only one of them has an ok roll. The other one isn't even as good as the one from collections.
The only reason I do these anymore is to get the new exotic each season, and that should be a guaranteed drop on your first completion with each character.
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3 Repliesit has 25% drop rate your just unlucky