Recently I was working with one of the Drupal 8 sites and faced with this issue of disappearing block.
This happened to new custom blocks that was created and placed in a block area or the existing blocks that was removed and added back. It never showed up
I have tried many of the suggestions including running the command
drush sqlq "update block_content_field_data set reusable = 1;", but nothing happened.
After spending several hours, found the issue. It was associated with the rules module that was enabled in the site. After uninstalling the rules modules, and re-introducing the custom blocks into the block area, everything started working.
Hope this helps someone.