Hey,
Good chance the postmaster just became full and they were cycled out, once gone you wont be able to reset/recover them.
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Well that's not exactly a good system for keeping a hold of materials you need weekly exotics and master working.. I've saved them for the sake of the last 10 exotics I've got from the vault to go
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Unfortunately the postmaster can only have a limited amount of space or else you will have postmasters thousands and thousands of items full, so it needs to cycle items out to make room for new ones.