Greetings!
I know there is a visual glitch where this happens with Enhancement Cores, maybe this is the same situation but with a weapon mod? I'm not too sure because I haven't seen it happen with a weapon mod before, but I thought I'd mention it. If this is the case, no actual Exotic was lost. [quote]EXOTIC ENHANCEMENT CORE
Enhancement Cores that are sent to the Postmaster will appear in the Postmaster’s inventory as Exotic engrams with zero Power. Collecting this Exotic engram is intended to only award a single Enhancement Core, and players should be aware that they are not missing out on a real Exotic engram reward. [/quote]
https://www.bungie.net/en/News/Article/50209
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That could possibly be it, I've never personally had anything like this though I seem to always be good at glitching things in accident XD. Since it had a power lvl and things have been fine in my game I wouldn't have thought of anything like this but since it can happen with cores could be linked to the same thing.