I was going to say don't worry about it, since Animmaniac's fix works, but after porting it into the full project, I just realized that it actually doesn't...
Alrighty, explanation time. The ninja, as you see, is just a blob, so most of his animations are him bouncing around like a cartoon ball. The previous animation system used frames to do all of this, but the new one uses far less frames and instead stretches and squishes him by changing width/height with events. Unfortunately, that makes it a lot more difficult to use mirroring; since that works (presumably?) by making the width negative, if you use "Set width" and set it to a positive number, it messes things up. I couldn't get that working right, so rather than use mirroring to make him face left, I used a second frame and flipped it in the editor.
This is the basic animation system, with everything but walking and turning cut out. Hopefully you'll see what I mean: