[radarhere]

#10061 (comment) found that

from PIL import Image, PngImagePlugin
im = Image.new("L", (8, 8))
im.save("out.png", save_all=True, disposal=PngImagePlugin.Disposal.OP_BACKGROUND, append_images=[im.copy()])

raises

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "PIL/Image.py", line 2712, in save
    save_handler(self, fp, filename)
  File "PIL/PngImagePlugin.py", line 1349, in _save_all
    _save(im, fp, filename, save_all=True)
  File "PIL/PngImagePlugin.py", line 1555, in _save
    single_im = _write_multiple_frames(
                ^^^^^^^^^^^^^^^^^^^^^^^
  File "PIL/PngImagePlugin.py", line 1249, in _write_multiple_frames
    base_im.paste(dispose, bbox)
  File "PIL/Image.py", line 1935, in paste
    self.im.paste(source, box)
ValueError: images do not match

The solution is simply to ensure that PngImagePlugin converts the image internally when saving.