Spigot 1.15.2
ItemJoin 5.0.6-b366
WeaponHolder 1.5

This may well be a WeaponHolder issue, but I'll start here to verify that it isn't an IJ issue.

If I (accidentally) place a locked IJ item on a WeaponHolder tripwire-hook, it accepts the item (ie., it is removed from inventory slot and placed on the tripwire-hook). When I take the item back, it is locked into a different inventory slot (the first available, left-to-right).

items.yml

  changeWorld-tool:
    id: CLOCK
    slot: 8
    count: 1
    name: '&bNavigate'
    lore:
      - 'Teleport elsewhere'
    commands:
      right-click:
        - 'deluxemenus open navigation'
    enchantment: FIRE_ASPECT:1
    itemflags: cancel-events, death-drops, hide-attributes, hide-durability, inventory-modify, placement, self-drops, unbreakable

So question, is this item suppose to be able to be placed into the WeaponHolder tripwire hook or is it suppose to stay in the player's inventory?

As for it being placed in the first available slot -- spigot does that so it's completely normal. To put it back in its normal slot I would have to listen to pickup events, remove the item and regive the item again.

There look to be a number of major problems with WeaponHolder. Among them, WeaponHolder will accept a broad range to items, without configurable limitation. I have filed several issues there. Until some of them are resolved, I will disable it.

Pending limitation on item types that WeaponHolder will accept, I suppose an alternative would be for it to (somehow) know about slot-locked items.

Unless you think differently, I'm willing to close this issue, since these are clearly WeaponHolder problems.

So after some testing from what I can tell, it is most likely an issue with WeaponHolder. I tried various ways to cancel the action but it seems like WeaponHolder overrides it. It's hard to tell though because WeaponHolder has some bugs that make it hard to know if it's attempting to cancel an action when it throws error codes.

Thanks. As I said, I have posted several issues with WeaponHolder, include the bugs that throw errors. We'll see how they respond. Truth is, the plugin is that that important, especially when it doesn't work on my servers.