Hi there,
Reusable block is a WordPress core feature beyond the theme’s control. There are known bugs related to it but we can’t quite fix it up if the issue with with actual WordPress core itself.
One example of a known bug is if the page you’ve created the reusable block on is deleted.
If the page you’ve created the reusable block on is deleted, the saved reusable block won’t show up on the list of reusable blocks on the Reusable blocks tab even if you find it on the Manage reusable blocks page. /wp-admin/edit.php?post_type=wp_block
In this case you’ll have restore the deleted page orr to re-do the block and make sure the page where you’ve created it isn’t deleted so you can keep using the reusable block.