• The items on your hand will disappear (also run commands)
• After the item on the hand disappears, the item on the offhand will disappear.

Video
https://www.youtube.com/watch?v=2ZA_IWjNoqc&t=0s&list=PLdb2DzWz3uEgL2RjXZTXZaAkk3NV-R4J4&index=8

config.yml
https://pastebin.com/KCd4SfYy

test_item2:
id: STONE
slot: Arbitrary
name: 'Test Item2'
lore:
- 'disposable'
- 'inventory commands'
commands:
inventory:
- 'message: &finventory click &a✔'
commands-type: inventory
itemflags: disposable
triggers: disabled
permission-node: 'itemjoin.test_item2'
enabled-worlds: All
test_item3:
id: GLASS
slot: Offhand
name: 'Test Item3'
lore:
- 'offhand'
- 'disposable'
- 'inventory commands'
commands:
inventory:
- 'message: &finventory click &a✔'
commands-type: inventory
itemflags: disposable
triggers: disabled
permission-node: 'itemjoin.test_item3'
enabled-worlds: All

ItemJoin version: 5.0.1
Spigot version: git-Spigot-19c3c5a-5932f8a (MC: 1.13.2) (Implementing API version 1.13.2-R0.1-SNAPSHOT) (2 version(s) behind)

Might I say you a really clever when finding these bugs haha, some things I would have never thought of.

Also tested an seems to be fixed, in the latest snapshot (along with the other bug post); http://ci.craftationgaming.com/job/ItemJoin/186/

This issue has been officially fixed in the release of ItemJoin v5.0.2.
You can grab it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=258611

Please read the changelog as the documentation has not been updated yet;
https://www.spigotmc.org/wiki/recent-changes