Saving png sequence as WebP in pil makes background opaque black

from PIL import Image, ImageSequence


def gen_frame(im):
    alpha = im.getchannel('A')
    # Convert the image into P mode but only use 255 colors in the palette out of 256
    im = im.convert('RGB').convert('P', palette=Image.ADAPTIVE, colors=255)
    # Set all pixel values below 128 to 255 , and the rest to 0
    mask = Image.eval(alpha, lambda a: 255 if a <= 128 else 0)
    # Paste the color of index 255 and use alpha as a mask
    im.paste(255, mask)
    # The transparency index is 255
    im.info['transparency'] = 255
    return im


im = Image.open("input_gif.gif")

im_list = []

for frame in ImageSequence.Iterator(im):
    # perform some functions on the image frames
    
    # frame = frame.convert('RGBA').resize((512, 512), Image.ANTIALIAS)
    # frame = gen_frame(frame)
    
    im_list.append(frame)

img = im_list[0]
imgs = im_list[1:]

img.save("output_gif.gif", save_all=True, append_images=imgs, duration=100, loop=0, optimize=False, disposal=2)
# works correctly, as intended

img.save("output_webp.webp", save_all=True, append_images=imgs, duration=100, loop=0, optimize=False, disposal=2)
# Transparency loss in the webp format

when saving as gif, it works great, but when saving as webp, the transparency is lost in all frames.

output of gif:

in webp its same, just no transparency, all black background

how can this be fixed?

from Saving png sequence as WebP in pil makes background opaque black