When adding an attachable file, that changes the model and texture of the held item, to the snowball, ender pearl, and egg it affects the projectlie and not the item in hand. When coding the attachable differently it does add the custom item model, but the original item is visible.

The attachable was added using a resource pack.

To replicate this, you need a resource pack that adds an attachable to the snowball, ender pearl, or egg item that gives it a custom model when held.

The issue may be caused by the listed items' identifiers being the same as their entity projectile counterpart.