This is kind of hard to describe, but I noticed this bug in my survival world (on my bds) , after building arrow slits for my castle, I then hopped in to a local creative world and replicated the same issue for the screenshots provided below.

1, Place a stair block sideways so you can see the 1/4 block gap, you can shoot through this.

2, Place another stair block in front of that block at 90 degrees on the x axis, the front stair block will morph/connect with the rear block in a way that cuts off half of the top portion of the stair block so you can still see through the gap in the rear stair, you can’t shoot through this gap now, the arrow or trident just hits the gap and stops