It just seems really petty. Especially calling it a "bug" when it's clearly a nerf. It just adds insult to injury.
If you pay the magicka cost you should at least be guaranteed the 4 seconds of major expedition, even if you end the channel early.
It really hurts vampire, which is already hard to justify in PvP. If you nerf Elusive Mist like that, then dropping vamp and going with the improved Race Against Time is just a no-brainer.