Short description of Custom Model Data: 

Custom Model Data allows resource packs to add additional textures and models to Minecraft items without replacing the default textures and models. These can then be enabled by spawning an item with a custom_model_data Data component. If the player has a resource pack enabled that matches the passed string in the custom_model_data component, then the texture and/or model is rendered.

Summary:

Attempting to spawn an item with custom model data from a resource pack does not work on 1.21.5 Spigot servers. Some testing has shown that it functions properly in single player and in a vanilla 1.21.5 server. Interestingly, custom sounds DO function properly.

To recreate:

Download and enable the attached resource pack, enter a 1.21.5 Spigot server and run the following command:  /give @p minecraft:stick[minecraft:custom_model_data={strings:['bobby']}]

This is supposed to result in the player receiving a stick that is textured as a small picture of Bobby Hill, but currently only spawns a stick with a normal stick texture.

This is my first time submitting a bug report so apologies if I have done anything incorrectly. Please let me know if there's anything I can improve or clarify. Thank you Spigot team for all your hard work!