I opened a prime engram from the Cryptarch, i got an exotic engram which got sent to the postmaster.
When i went to the post master and claimed the exotic engram it was power 0 and just gave me an enhancement core.
The same thing happened to my friend, but instead of an enhancment core he got a helmet armor mod.
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There is a bug that makes enhancement cores and mods drop as 0-power 'Encrypted Engrams', when your engram inventory is full. This can happen when you open Lost Sector chests, complete Nightfalls or decrypt a Prime Engram (when the other nine slots are full). It is a known issue and is just a visual bug. It was just a core or mod all along and did not take the place of anything rarer.