They still haven't fixed this?!?!???
Like Jay said, this became a problem once upgrading to CC. It cripples such a vital part of the coloring process, and also ruins the work on all my previous projects, which once worked fine. When I noticed this problem 5 months ago, I tried posting numerous bug reports, and I also reproduced the bug several times with Adobe Tech Support. They told me they would fix it, but apparently they didn't.
Through many days of troubleshooting, I came to realize that masks don't work when your clip involves scaling. This bug renders speedgrade absolutely useless since 99% of the time, I'm working with RAW 5K Red camera footage, scaled to fit a smaller timeline. On the phone with tech support, we tried all sorts of workarounds, until I finally gave up. Come on Adobe.... don't you know that it's important for colorists to have the ability to create masks??? This is such an embarrassment!
For now, my suggestion is to instead use the industry standard application, DaVinci Resolve. It's infinitely better, not to mention FREE.
In case you're interested, here are a couple of my previous threads on the issue:
https://forums.adobe.com/message/6245538#6245538#6245538#6245538
https://forums.adobe.com/message/6247527#6247527#6247527#6247527