Those are inbetween frames, they are intentionally drawn like that and not noticeable in motion. It's neither a mistake nor is it bad art.I wonder: how the hell stuff like this happens? This isn't some computer generated thing - someone must have deliberately draw it that way. And even if it's a transition frame, it's still weird and once you notice it, you can see that it looks out of place even during the animation.
I think Crystals biggest problem at the moment is the wait between episodes, it's so serialized that the episodes can hardly stand on their own, it worked during the early part of the arc when each episode focused on a senshi but now it feels like they hacked a movie into pieces.
Crystal is one of those shows that will be appreaciated more if people can binge it imo.