/damage doesn't damage held shield

If you use the /damage command on a player while they're using a shield, the "block attack" sound will play, but the shield will not get damaged or play the "got attacked" animation.

 

How to reproduce:

• Set up a command block that hurts the player with mob_attack by an entity that is in front of them (e.g. "damage @p 1 mob_attack by @e[type=!player,sort=nearest,limit=1]"){}
• Give the player a shield
• Hold right-click while holding the shield

What is expected to happen:

• Hurt the shield's durability and lower the model in first person, like a normal attack

What happens instead:

• The shield sound plays, but it doesn't get damaged