A shield with an specified item model other than default won't show the blocking animation in first person, despite this working as expected on other items with an specified blocks_attacks component.

How to reproduce

1. Give yourself a shield with a different item model:

   /give @s shield[minecraft:item_model="blue_stained_glass_pane"]
   

2. Now give yourself another item but with the blocks_attacks component:

   /give @s minecraft:blue_stained_glass_pane[minecraft:blocks_attacks={}]
   

3. Compare the blocking animation with both items.

Expected result
Blue stained glass pane with the blocks_attacks component.

Observed result
Shield with the item model of a blue stained glass pane.