its an old bug that when the tnt flies through unloaded chunks it becomes frozen as a entity
it must of been that the items may had despawned as you might have been close to the items as they were loaded it takes the game 5 minutes for them to despawn
no seen some in my survival world on Sunday they are rare so might be that
no its a trophy for defeating the wither for the first time and it may be that the world he is playing on has cheats or he may have the trophy all ready.