When getting the 17th Mote for the Exotic Khvostov there is a chance that all of the motes disappear instead of giving the Exotic Mote. There isn't any way to claim the exotic mote either so they are all just lost. I hope this gets fixed soon because I was looking forward to the weapon.
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Well for my case, I had the 16, but I did not do the boss. I just did the cyst of the seclusion, got the exotic mote, and I was not able to open the golden chest...