I have been searching for a long time but could not find the reason. I noticed it today. When I prepare the block in sketch environment and save it in sketch environment, the preview images do not appear. But when I prepare it in sketch or drw and save it in technical drawing page, the preview images appear. Look at the details in the images.
I am sharing it here to guide those who have the same problem. I don't know, maybe most of them know this, but I noticed it today by coincidence.