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When a respawn anchor is covered on all sides (including diagonals) either with solid blocks or air, it will count as being obstructed, even if there would be space above the anchor.
Respawn anchors in midair are considered obstructed
respawn anchors with no blocks around it don't work
You respawn next to the respawn anchor instead of on top of it
Respawn anchor resets the player's spawn point to the world spawn point if it is obstructed
Carpets obstruct respawn anchors