E-Ink Display shows weird behaviour with horizontal lines --> No contrast

My E-Ink display (Waveshare E-Ink, 7.5 inch, Version 1, 640×384) connected to Raspberry Pi Zero W via the e-paper HAT (https://www.waveshare.com/wiki/7.5inch_e-Paper_HAT) shows some content with horizontal lines very weak. Other content is displayed correct. Any suggestions how to solve the problem?

Grid (with horizontal and vertical lines) is rendered via PIL (see code):

The above image drawn on the E-Ink display (if you look carefully you see the horizontal lines very weak at the bottom):

Another image: horizontal lines fade away in the top right corner:

Another image: horizontal lines drawn with no gaps so that an all black image is drawn. No contrast is lost here:

My hardware setup - Raspberry Pi Zero W with the e-paper HAT:

I use the following python code to display the images

from PIL import Image
from PIL import ImageDraw

EPD_WIDTH = 640
EPD_HEIGHT = 384

bl = 0

def main():
    image = Image.new('1', (EPD_WIDTH, EPD_HEIGHT), 1)    # 1: clear the frame, 1: all black
    draw = ImageDraw.Draw(image)
    for x in range(35):
        for y in range(384):
            draw.rectangle((0, y*20, EPD_WIDTH, y*20+2), fill = bl)
            draw.rectangle((x*20, 0, x*20+2, EPD_HEIGHT), fill = bl)
    try:
        from waveshare_epd import epd7in5
        epd = epd7in5.EPD()
        epd.init()
        epd.display(epd.getbuffer(image))
        epd.sleep()
    except:
        image.show()
        image.save("frame.bmp")
if __name__ == '__main__':
    main()

epd7in5 modules from: https://github.com/waveshare/e-Paper