Player Head Items Not Stacking

Summary

Mythic items using the player_head base item no longer stack. No relevant startup errors or errors when the mm item command is executed.

Steps to reproduce

1. Create base player head item--doesn't seem to matter how complex, but all of the ones I've tried did include textures. The texture itself doesn't seem to matter.
2. Use /mm items give [item name] 5 (e.g.) or collect multiple of the item through mob loot.
3. Weep as your inventory fills with unstackable heads.

Intended correct behavior

Player heads of the same type should stack - this is vanilla behavior, stackable to 64.

Version

Using MythicMobs-5.11.0-SNAPSHOT-b5572 with MythicCrucible-2.2.0-SNAPSHOT-b4057 (the presence of Crucible doesn't seem to be a factor) on Paper version 1.21.10-113-main@9fc21bc.

I'm not entirely sure when we first noticed the issue, but I believe somewhere around build 5566.