This is a known issue Bungie is investigating.
https://www.bungie.net/en/Forums/Post/260156995?sort=0&page=0
[quote]Players are unable to reclaim the Strange Key if it is deleted, blocking progress for the Magnum Opus Exotic quest.[/quote]
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Ahh, Thank you. I didn't even notice that posting. Ill be sure to check for that next time