The problem here is that your “left” and “right” animations have too much “forward” in them. The player isn’t literally sidestepping, he’s playing a forward-walk-type animation which is oriented leftward/rightward.
Your blend is busted because the engine is trying to figure out what halfway between “walking left while facing left” and “walking backward facing forward” would be, and the result is not “walking back-left facing forward-right” as you seem to expect.
There are two options here; one is to create a version of your left and right walk anims which are backward (i.e. moving left by backing-up-from-the-right and vice versa) so the anims are close enough to yield a logical middle ground.
The other is to do what I do in my game, and only create TWO walk anims, a forward and backward, and choose which one to play based on whether the player is moving toward or away from the front hemisphere of the control rotation, and then use aim offsets to twist the torso into a “looking forward” pose.