I'm using sophisticated backpack in 1.18.1 server and after looting a dungeon from "when dungeon arise" the diamond backpack inventory turn into decrease and only have 27 slots on it.

Any answer to my questions? Or should I just close?

closing given no additional feedback provided

If you craft a new diamond backpack does it have 27 slots as well? and are you saying that you have found diamond backpack in the loot there? I am not even sure that I am including diamond backpack anywhere apart from maybe end loot.