Since the 1.19.4 update, explosion knockback has been affected/decreased. If you are wearing 1 or more blast protection piece of armor it will reduce knockback a lot, the same amount as if you had 4 pieces of armor with blast protection.
Linked issues
relates to 1
Comments 5
That doesn't mean it's intended that it's not reduced further if you have several pieces with it. The effect was made more visible on lower levels, but that effectively made stacking not function.
I suggest a reopen and relates to link instead.
This issue was fixed in 1.21rc1. It says, "Knockback reduction from Blast Protection still stacks from wearing multiple pieces (changed from 1.20.6)."
Works As Intended as per the fix of MC-198809.