I was unable to get a screen recording, however I can attempt to describe it. This happens anywhere you're trying to get motes in the Prophecy dungeon, most notably in the Kell Echo encounter. I was running Arcstrider Hunter. Every time I pick up 5 stacks of either light or dark motes, the stacks immediately disappear. I made sure I wasn't dodging when picking them up too just to be sure, and they still vanish from my buffs and debuffs sidebar after about a half a second of saying "Light/Dark Motes Max." I'm picking them up well within the timeframe as well. If someone is able to potentially recreate this issue, I'd greatly appreciate it!