I am seeing the same in my RCON. Was just about to post, but will watch this one instead :)

For anyone who wants to test my solution until the plugin gets updated:

        private void OnPlayerDisconnected(BasePlayer player)
        {
            if (_openBackpacks.ContainsKey(player))
                _openBackpacks.Remove(player);

            if (!_config.SaveBackpacksOnServerSave && _backpacks.ContainsKey(player.userID))
            {
				var backpack = Backpack.Get(player.userID);
				
                _backpacks[player.userID].SaveData();
                _backpacks.Remove(player.userID);
				backpack.DeInitialize();
            }
        }


            public void DeInitialize()
            {
                if (_initialized)
                {
					_itemContainer.Kill();
                }
            }
​

Added DeInitialize in Backpack class and added a call for DeInitialize in OnPlayerDisconnected.
The ghosty items which are already there won't go away with this change. But no more items will get added.
To get the items which are already there away, the only solution I know is dying -> disconnect -> connect -> respawn.

Version 3.0.1 includes a fix for this issue.
Thank you for the details to reproducing the issue.