Pillow Black&White Image is saved as a blacksquare - image

Question

I get an image from PySide6 Canvas.
I can show it in the browser properly. But while saving this one, I get the black square.
While converting the image into a NumPy array I get correct figures in the array.
Large screenshot of Numpy array
I can't understand what is going wrong while saving image.
I expected to save the image properly but I can't find a way to.
the class code
import numpy as np
from PIL import Image, ImageOps
from numpy.typing import NDArray
from typing import Tuple, Union
THRESHOLD = 100
FINAL_SIZE = 28
class Utils:
#classmethod
def get_formatted_image(cls, pil_im: Image, as_ndarray=True) -> Union[Image.Image, NDArray]:
"""
Converts an image of the number into NumPy array
"""
pil_im = cls._get_cropped_image(cls._get_gscale_image(pil_im))
pil_im = pil_im.resize((FINAL_SIZE, FINAL_SIZE), resample=1)
pil_im = cls._get_gscale_image(pil_im, inverse=False)
return np.array(pil_im) if as_ndarray else pil_im
#staticmethod
def _get_gscale_image(img: Image, inverse=True) -> Image:
"""
Converts image into a black-white one
"""
converted_image = img.convert('L')
return ImageOps.invert(converted_image) if inverse else converted_image
#classmethod
def _get_cropped_image(cls, img: Image) -> Image:
"""
Crops the image (converts the image into a squared one)
"""
img_np = np.array(img)
boundaries = cls._get_boundaries(img_np)
img_np = img_np[boundaries[0]: boundaries[2], boundaries[1]: boundaries[3]]
return Image.fromarray(img_np)
#staticmethod
def _get_boundaries(np_img: NDArray, inverted_image=True) -> Tuple:
"""
Gets boundaries of the image
"""
x = {k: v for k, v in enumerate(np_img.sum(axis=1) / np_img.shape[1])}
y = {k: v for k, v in enumerate(np_img.sum(axis=0) / np_img.shape[0])}
if inverted_image:
x = list(filter(lambda z: z[1] > 0, x.items()))
y = list(filter(lambda z: z[1] > 0, y.items()))
else:
x = list(filter(lambda z: z[1] < 255, x.items()))
y = list(filter(lambda z: z[1] < 255, y.items()))
return min(x)[0], min(y)[0], max(x)[0], max(y)[0]
Anything else is given above. There is nothing else I can add.
processed_own_image.show()
gives expected result, but
processes_own_image.save('temp.png', 'PNG') gives a black square