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I think that's because you're so close to the shooter.
Since laser blade protection is similar to vanilla shield, it takes 0.25 seconds to prepare blocking. An arrow travels about 15 blocks in 0.25 seconds, so we need to press the right mouse button while the arrow is farther than that distance. For example, when fighting a skeleton that shoots arrows from a distance of 15 blocks, we need to press the right mouse button before it shoots an arrow.
I won't fix this to keep the laser blade protection the same as vanilla shield.