cv2 is bgr whereas matplotlib is rgb

[samrere]

There's a tiny mistake in the function show_mask_on_image. The heatmap you get from cv2 is in bgr format, so you need to convert it to rgb before adding to the img:

def show_mask_on_image(img, mask):
    ...
    heatmap = cv2.cvtColor(heatmap, cv2.COLOR_BGR2RGB)
    cam = heatmap + np.float32(img)
    ...