Everything is part of the Force in Star Wars. Literally everything. So when Luke faded into the Force, all of what Luke considered to be "him" faded with him. Same reason most of Obi-Wan's clothes other than his outer robe and lightsaber vanished in ANH. Luke had his cybernetic hand for so long that he considered it a part of himself, so it went with him into the Force.
If a Jedi were killed by a lightsaber (like Obi-Wan was) super fast, would every body part vanish?
If the Jedi has their limbs cut off first, then they were killed, would the limbs still vanish?
If a Jedi donates blood, and the Jedi is killed, would their donated blood vanish from the patient it was given to?
If a Jedi swallows a massive load of cum from someone else, and the Jedi dies, does the cum vanish with them, or would it stay behind and splat to the ground since it's not really apart of them?
If a Jedi was filming a 100-man bukake scene, and then dies...