On my baby rogue, here's what happened:
Backstab was lowered to level 1. So you can raise it from level 1 on, right? I got it up to ~25 or so, at level 6. As soon as I hit level 7, on my skills window, it says that it's 0. At first, it had a RED 0, but now it's just a white one. Bug, or is it just not displaying my skill level properly?