In previous versions of Minecraft, using multiple Blast Protection enchantments caused no stacking effect. In 1.21+ the enchantments stack and cause people to take absolutely no explosion knockback.

Steps to reproduce:

• Enchant multiple Netherite gear with Blast Protection IV (e.g chest/leggings)
• TNT or end crystal
• Observe there is no knockback

While this may be expected behaviour it certainly does not feel so.