Convert all png images in current folder to jpg using OpenCV cv2

png2jpg.py

#!/usr/bin/env python
from glob import glob                                                           
import cv2 
pngs = glob('./*.png')

for j in pngs:
    img = cv2.imread(j)
    cv2.imwrite(j[:-3] + 'jpg', img)

I found same strange bug and using this script. I try to convert 4k image png(sizes almost 40mb) to jpg.
color image but Normal map image works fine.

this are strange effect after running scripts

I think first image conversion has error other are fine atleast to my eye vision.
if any one notices difference in below images too , plz kindly report by commenting.
thank you in advance.