The bug

Hitting a target block from exactly 30 blocks away while brushing up against a block right in front of you counts as being less than 30 blocks away and doesn't grant the advancement "Bullseye."

How to reproduce:

1. Place a target.
2. Place a redstone wire that extends 15 blocks off one side of the target.
3. Place a command block at the end of the wire with the following command:

   /say 15

4. 29 blocks away from the target, place a solid block.
5. Stand behind that block, so that you are 30 blocks away from the target.
6. Walk forward so that your hitbox collides with the block.
7. Continuously shoot at the target, trying to hit the exact center, until you see the message [@] 15 in the chat.
   → You did not obtain the advancement "Bullseye."

Expected result:

The player should obtain the advancement "Bullseye" since they were 30 blocks away from the target when they shot at it.

Observed result:

The player did not get the advancement "Bullseye" despite being 30 blocks away from the target.