The bug:

Using an item modifier to modify CustomModelData on an item in an item frame doesn't visually update the model. The NBT does get copied to the item from the item modifier correctly, but to get the model to update, you need reload the world or modify some NBT on the item frame.

Steps to reproduce:

1. Add the data pack to your world and apply the resource pack
2. Run /function test:setup to setup the NBT storage location and to give yourself an item frame and an item that can use the CustomModelData model predicate
3. Place the item frame down and put the cake item in it
4. Run /item entity @e[type=minecraft:item_frame] container.0 modify test:set_model
5. If this was working correctly, the cake item should have changed into a golden apple
6. Reload the world or modify some NBT on the item frame and you'll see the model change into a golden apple