Why do the photos have a distinctive horizontal line after being edited in mosaic (the third effect) and stored?

The mosaic significantly reduces the fine details in the photo. Small features and textures become obscured, resulting in a simplified, abstract representation of the original image.
The original image needs to be output when saving after being edited in mosaic, it will be processed in blocks when the image is large. After the blur effect is processed, there will be a boundary line when the spliced parts are jointed together. This will not affect your use. We will try to continue to optimize such scenarios and improve your experience in the future.