well, the “designer” at the desk next to you might be an idiot, but you can still “repair” it:
im = cv2.imread("img.png",-1) # load WITH alpha
alpha = im[:,:,3] # extract A
im[alpha==0] = 255 # reset to 'opaque'
i2 = cv2.cvtColor(im,cv2.COLOR_BGR2GRAY) # there we go !