SET's mechanic doesn't register offhand items as part of set

Server Version Paper version 1.21.10-113-main@9fc21bc (2025-11-14T16:11:13Z)

Plugin Version(s) ProtocolLib v5.4.1-SNAPSHOT-5773bcb MythicMobs v5.11.0-SNAPSHOT-4b0382d7 ModelEngine vR4.0.9

Issue description: If you add set item for offhand (f.e. shield), take it to offhand - it wont be registered as part of set so effects from set won't work.

Steps to reproduce:

  1. Use this amazing set at your server:
BABY_BOOBOO:
  Enabled: true
  Display: 'Baby Booboo Set'
  Lore: 
  - '<#FF9999>Baby Booboo Set<&co>'
  - '<#FF9999>Makes you small and fast'
  Bonuses:
  - Pieces: 2
    Stats:
    - SCALE 0.5 SETTER
    - MOVEMENT_SPEED 1.2 MULTIPLY
  1. Add two items for test:
test_iron_sword:
  Id: iron_sword
  EquipmentSet: BABY_BOOBOO
test_iron_shield:
  Id: SHIELD
  Material: SHIELD
  EquipmentSet: BABY_BOOBOO
  1. Take items in main and offhand and see nothing. Or even you may take two swords in both hands, which also should work cause we don't have any conditions for sets equipslot.

image

Feel free to ask more details in DS: @DezForse

Thank you!