Um, don't worry about it? I have noticed lag when decrypting engrams occasionally; I guess sometimes things can fail to decrypt entirely. Even if you got a legendary from that engram, it's not as good as new raid weapons or an exotic, so there's no reason to be upset. And if it was a rare, even less so.
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Ahhhh ok oh well shitty but it is a game not worth getting too worked up over minuscule crap.