Off hand gets deleted when tool in main hand breaks when hitting a mob and you replace the item in the main hand at the same time

Expected behavior

Tool breaks and is replaced by the new item

Observed/Actual behavior

tool gets replaced, but itemstack in off hand gets deleted

Steps/models to reproduce

`

@EventHandler
  public void onPlayerJoin(PlayerJoinEvent event) {
    Player player = event.getPlayer();

    ItemStack tool = new ItemStack(Material.STONE_SWORD); // Any tool works, as long as you are hitting a mob

    Damageable meta = (Damageable) tool.getItemMeta();

    assert meta != null;
    meta.setDamage(10000); // Making it break in one hit; it does not need to be over the limit

    tool.setItemMeta(meta);

    player.getInventory().setItemInMainHand(tool);
    player.getInventory().setItemInOffHand(new ItemStack(Material.GRASS_BLOCK)); // this item does not matter either, just placeholder
    player.getWorld().spawn(player.getLocation(), Chicken.class); // entity type does not seem to matter
}

@EventHandler
// PlayerItemDamageEvent also works, so long as the tool is replaced immediately as it is broken.
public void onToolBreak(PlayerItemBreakEvent event) {

    Player player = event.getPlayer();
    int slot = player.getInventory().getHeldItemSlot();
    player.getInventory().setItem(slot, new ItemStack(Material.STONE)); // replacement item does not matter

    // off hand item has now disappeared.

}

`

Plugin and Datapack List

no plugins besides one with the above code is required

Paper version

This server is running Paper version 1.21.8-56-main@65641d1 (2025-08-29T20:44:17Z) (Implementing API version 1.21.8-R0.1-SNAPSHOT)
You are running the latest version
Previous version: 1.21.8-31-86b716b (MC: 1.21.8)

Other

this issue goes back several patches, perhaps full minecraft versions. I first noticed it as a paper bug on my 1.21.4 server.

Reproduce the issue using the provided PlayerJoinEvent setup and PlayerItemBreakEvent handler on Paper 1.21.8. Start by tracing item-break and inventory handling when the main-hand slot is replaced, then verify that the off-hand ItemStack remains present after the tool breaks. The issue provides no named source files or tests.