In brief summary:

Addon armors that include the component "minecraft:knockback_resistance" do not apply any amount of knockback resistance to the armor.

Screen_Recording_20240221_211401_Minecraft[1].mp4
In the linked video above testing in regard to this issue is done as follows:

• vanilla netherite armor (+1 knockback resistance per piece)
• addon armor (intended to be +2 knockback resistance per piece)

The player being tested on is also using Minecraft for Windows as well.

The addon armor provided absolutely no protection from knockback in this testing.
In the previous update, these armors were fully functional in regards their knockback resistance.

Thank you for your work at keeping this game fully functional.