addItem problem with offhand and full inventory

[ssomar1607]

Expected behavior

Hello.

The following method when there is a space to add the item should add the item in the inventory:

PlayerInventory inv = p.getInventory();
inv.addItem(new ItemStack(Material.DIAMOND));

On my screenshot, I have space in my offhand, so it should increase to 37, 38, 39 up to 64.

Observed/Actual behavior

But currently, it doesnt work when the space is in the off hand.
The additem method return a non empty Map.

Steps/models to reproduce

Create a small plugin with

PlayerInventory inv = p.getInventory();
inv.addItem(new ItemStack(Material.DIAMOND));

when you do a specific action

put at least one of the item you add in your offhand and fullfill your inventory with another item

Plugin and Datapack List

[21:25:21 INFO]: - AceStore, CustomPiglinsTrades, ExecutableBlocks, ExecutableEvents, ExecutableItems, LuckPerms, MyFurniture, NBTAPI, packetevents, PlaceholderAPI
[21:25:21 INFO]: SCore, Vault, WorldEdit

Paper version

[21:25:29 INFO]: This server is running Paper version 1.21.4-76-main@486e521 (2025-01-05T18:15:54Z) (Implementing API version 1.21.4-R0.1-SNAPSHOT)
You are running the latest version
Previous version: 1.21.4-53-326c687 (MC: 1.21.4)

Other

No response