Mist Form is removing the 6 second immunity that you receive after being hard cc'd. I'm convinced this is a bug, though the vague wording in the ability's tooltip had me second guessing whether it was a stealth nerf or not. Basically, if you get stunned, then break out, then mist for a second, then exit mist form-- you are instantly able to be stunned again, even if has only been a couple seconds. This creates an issue when trying to use Mist Form as a repositioning tool.