To place bookshelves for the enchantment table, you only need 15 to add the Enchantment table to a full level. However, when I put the bookshelf in my survival archive, I used 32 bookshelves to surround the enchantment table, but the bonus was only 24 levels.

I changed the enchantment table to a different position, and the number of bookshelves I needed changed back to 15.
It is difficult to reproduce the steps because it takes luck to trigger it
 Step reproduction:
 1. Place an enchantment table.
 2. Place bookshelves around the enchantment table. 3. Open the Enchantment table.
If you are lucky, you will find that the Enchantment table has not been added to level 30. (Triggering this bug may be related to coordinates, and in the same archive, the same coordinate can always trigger the bug.) But in different archives, it's not related to coordinates)