Agreed, That happened to me once. I can understand no postmaster because it risks bugs of storing more than one cipher, but there should be a warning before you claim a cipher. This might not be a bug per se because it's been this way for a long time, but part of development is rational improvements.
Bungie has been adding tutorial notes on activities lately, maybe they will add some to Ciphers for unsuspecting players.
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I have just suffered this issue too. It’s a bit poor the game knowingly let’s the game lose an item. The fact I will have to grind again for another is frustrating. I agree with the OP, if there was a weapon quest and your slots were full it would either not let you turn it in or be sent to the postmaster. This bug needs to be fixed. I’m someone who fully believes in fairness at being rewarded the item you have worked towards.