It only happened in dungeon and tight zones so far.
If I mezz a running mob in SolA for exemple, he disappears and reappear when mezz is gone, then I can remezz him again and he does not go away again. (I think, they only poof into the void when they are being mezzed while running, not sure though).
I can say that this is a really really dangerous bug. Let me explain, you're in a hard dungeon spot, you pull, there's 2 adds, you mezz them but they both disapppear so you can't keep them mezzed, they reappear, hit/burn you, and your whole group is dead since you were supposed to control the crowd
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I'm only saying that to let you see what priority could this bug be.
Thanks to the dev team!