Brighter irregular edge of a Polaroid photo

I use Polaroid OneStep 2 with Color i-Type film packs.

Sometimes, at the bottom edge of the image, not fully developed irregular areas appear as the medium inside has not been properly distributed. Only the bottom edge is a problematic one, the rest of the photo is fine.

It does not seem to be related to the temperature the photos were taken in. Also, it has been happening with multiple packs, so it might not be the case either, I think. I try to keep the rollers clean every time I insert a new one.

What may be the cause of that? Below you can find a photo of the issue I’m talking about. See the bottom edge of the pictures, especially the second one.