ItemStack serialized to YAML with "internal" tag breaks on 1.20.6 > 1.21.1 update

Expected behavior

The "internal" tag will continue to work...

Observed/Actual behavior

The tag is lost upon loading and storing the ItemStack, as it now seems to be renamed to "custom" without automatic migration

Steps/models to reproduce

Given the below yaml output, load it as itemstack and save it again on 1.21.1

        spawns:
        - custom_egg:
            ==: org.bukkit.inventory.ItemStack
            v: 3700
            type: MAGMA_CUBE_SPAWN_EGG
            meta:
              ==: ItemMeta
              meta-type: SPAWN_EGG
              display-name: '{"extra":[{"italic":false,"color":"white","text":"Spawn LavaLurker"}],"text":""}'
              lore:
              - '{"extra":[{"bold":false,"italic":false,"underlined":false,"strikethrough":false,"obfuscated":false,"color":"gray","text":""}],"text":""}'
              - '{"extra":[{"bold":true,"italic":false,"color":"dark_green","text":"< "},{"bold":false,"italic":false,"color":"gray","text":"Left click for menu."}],"text":""}'
              - '{"extra":[{"bold":true,"italic":false,"color":"dark_green","text":"> "},{"bold":false,"italic":false,"color":"gray","text":"Right click to summon."}],"text":""}'
              enchants:
                DURABILITY: 1
              ItemFlags:
              - HIDE_ENCHANTS
              internal: H4sIAAAAAAAA/+NiYOBi4HbKLy6O90sqCUlM52BgB/PCTBi4fBLLEn1Ki7JTixgYADZ7h3UpAAAA

Plugin and Datapack List

N/A

Paper version

This server is running Paper version 1.21.1-85-master@c5a1066 (2024-09-19T14:47:47Z) (Implementing API version 1.21.1-R0.1-SNAPSHOT)

Other

I managed to "fix" this by manually replacing "internal" with "custom", however that should be dealt with automatically. CraftMetaItem seems to contain support for both, not sure why it no longer works.

Thanks!

Start with CraftMetaItem and the ItemStack YAML loading and saving path, using the supplied YAML and Paper 1.21.1 behavior as the reproduction case. Trace how the legacy "internal" tag and the "custom" tag are handled, then verify that loading and saving preserves the tag data without manual replacement.