Confirmed in 1.21.7
Steps to Reproduce:
1. Use Blockbench with Minecraft Item Wizard plugin to create a resource pack and behavior pack. the plugin can add these two packs to the developer resource pack folder and developer behavior pack folder.
2. Create a new item, like a chestplate, draw and paint...and save.
3. Open the game, add the resource pack and behavior pack to my world.
4. At the running time, open inventory, and you can see the bug,
Observed Results:
The chestplate up out my body, like my video. but the left and right arm of the chestplate model doesn't go up with the chestplate model. just chestplate has this bug, other slot armors display normally.
Expected Results:
every armors display is normal.
Confirmed in 1.21.8