I think it's sometimes, the character need to attack from another angle, like when you want to jump out of actual radius ; you'll see the "ghost" showing final position of your char before being able to trigger the real action (in your case, attacking from melee).
So I presume you didnt pay attention to this (sometimes it's unexpected and/or difficult to see because various obstructions of camera), and Lae'zel triggered an AoO with the slight move she had to do prior her attack ?
If not, yes AoO is currently a bit unreliable, and may lead to cancelling a character's action for some reason...
