According to the minecraft wiki explosion knockback resistance will only look at the armor with the highest explosion protection level on you.

If you have any piece of armor with explosion protection IV on you, your knockback will be reduced by 4*15%.

Hmm...it's a bit hard to describe
If you are wearing 4 pieces of armor with Explosion Protection IV your Explosion Resistance will be 0.6 (approximately), but if you take off one of them at this time
Will cause your explosion resistance attribute value to become 0, which will not reduce the explosion knockback.

Expected behavior:
If you drag off the armor with explosion protection, your explosion resistance will be updated to the value of the armor with the highest explosion protection level.

What actually happened:
Taking off the explosion protection armor will not update your values, but will directly lose your explosion resistance.

The translation may not be perfect so I recommend watching the video: