If the client has the "item_storage_size" set to 1 while the server has it set to 2 the first time you open the bag when moving to a new dimension will shift the inventory up by half a chest.
forthwist commented
This bug report just saved me so much ongoing frustration, thank you!
(Issue still occurring in EnderStorage-1.18.2-2.9.0.182-universal.jar)