I use an 18/2 rogue/fighter, which I recently GR'd from 15/5. I never used to use assassinate then, because my DC wasn't very high (32ish). Since I reincarnated, though, it's slightly higher, at 35. I thought I'd give it a try, so I went into the Vale wilderness area and tried to assassinate the rats. Went into sneak mode, dropped in behind one, and hit Assassinate - Whack, I hit him for two normal hits (with sneak attack). He didn't make any sort of save, though, but instead he and his buddies started swinging around at the air trying to find me. I waited until the timer was finished and tried again. Same thing, although this time he died on the first attack, but just from damage - no save was made or failed.

I did the same thing for the three other rats, two dogs, and a troll. No saves, ever. My Assassinate doesn't break Sneak mode, but it just hits for two normal/sneak attack hits. Any reason why this could be happening, or does it sound like a bug?

I've tried a few different set-ups, but for the most part I'm using a THF weapon, with Power Attack on. I'm always in Sneak mode and invisible, with a DC: 35 Assassinate.