Pillow image generation issue - merging multiple images

I had trouble finding a similar issue.

I am using Pillow (Python 3.5 on a MAC dev machine) to merge multiple images into a single long horizontal image.

The images are screengrabs saved as jpgs from Pyppeteer - that part works well, can open and see the image:

Below is the code I am using. The image_sets kwarg is essentially a list of dictionaries with a name of the image set and the set of saved images from the Pypetteer code, currently 2 sets for my dev code:

import sys
from PIL import Image

class ImageGenerator(object):

    def __init__(self, **kwargs):

        self.image_sets = kwargs['image_sets']

    def generate_images(self):

        for image_set in self.image_sets:

            #create PIL images from the saved images
            images = map(Image.open, image_set['generated'])
            widths, heights = zip(*(i.size for i in images))

            total_width = sum(widths)
            max_height = max(heights)

            new_im = Image.new('RGB', (total_width, max_height))

            x_offset = 0

            for im in images:
                new_im.paste(im, (x_offset, 0))
                x_offset += im.size[0]

            new_im.save("output/{}.jpg".format(image_set['set_name']))


if __name__ == "__main__":

    # NOTE - I removed code that was here, irrelevant to my question, but the
    # output is the "creatd_images" dict which I pass to the PIL class

    image_generator = ImageGenerator(image_sets=created_images)
    image_generator.generate_images()

This all works well - no errors. It creates images of the proper size and everything. The issue is when I open them, the new stitched together images are all black.

I have tried this with PNGs as well, same issue (this original image is like 15000px wide and 1080p high).

Here's the result: