When the player clicks in the hotbar inventory, checking the relative slots of InventoryClickEvent (#getSlot()) The slots do not map from 0 to 8 (like they should do) but they are (from right to left) [36, 37, 38, 39, 40, 0, 1, 2, 3]
This doesn't happen in any other vanilla inventory other than the player's normal inv.

code used to check:

@EventHandler(ignoreCancelled = true, priority = EventPriority.NORMAL)
protected void onPlayerClick(InventoryClickEvent e) {
    System.out.println(e.getSlotType().name() + ", raw:" + e.getRawSlot() + "->" + e.getSlot());
}//+ The rest of the listener, the main class for loading it and the plugin.yml file