After entering the server, the player receives information such as on ss:

My configuration:
items.yml - https://pastebin.com/2iXDf9Af
config.yml -

Console error - https://pastebin.com/raLi8VB0

Plugin version: 5.2.5-RELEASE-b850
Server: PurPur 1.18.2

Okay, so this now appears to be fixed in the latest snapshot.
If you continue to experience issues please send me your /itemjoin dump output and I will be able to progress further on a fix.

Please let me know!~
Link; https://ci.craftationgaming.com/job/ItemJoin/879/
Remember to remove your old ItemJoin.jar file