Yeah, it’s just a misalignment of the rotoscoping during the compositing process. I don’t know for certain exactly how it happens, but I’m pretty sure it’s just the various pieces of film used to create the composite image were not physically lined up well enough as they went through the optical printer.
Here’s an example of the same thing from an outtake of Star Wars:
In this case, it looks like they accidentally put the lightsaber-color film layer in flopped. (Like, it should be a mirror image of what we see here if it were going to properly line up with the blades).