Program: 11847
Project: 11847
Buggy source:
public static Player spawnPlayer(ClientConnection connection,UUID id){
  CompoundTag offlinePlayer=(OfflinePlayer.getOfflinePlayer(id) == null) ? null : OfflinePlayer.getOfflinePlayer(id).asNbt();
  if (offlinePlayer == null) {
    offlinePlayer=OfflinePlayer.generatePlayer(id);
  }
  final TridentPlayer p=EntityBuilder.create().uuid(id).spawnLocation(TridentServer.WORLD.spawnLocation()).executor(ThreadsHandler.playerExecutor()).build(TridentPlayer.class,ParameterValue.from(CompoundTag.class,offlinePlayer),ParameterValue.from(TridentWorld.class,TridentServer.WORLD),ParameterValue.from(ClientConnection.class,connection));
  p.executor.execute(new Runnable(){
    @Override public void run(){
      p.connection.sendPacket(new PacketPlayOutJoinGame().set("String_Node_Str",p.getId()).set("String_Node_Str",p.getGameMode()).set("String_Node_Str",p.getWorld().dimension()).set("String_Node_Str",p.getWorld().difficulty()).set("String_Node_Str",(short)10).set("String_Node_Str",LevelType.DEFAULT));
      p.connection.sendPacket(PacketPlayOutPluginMessage.VANILLA_CHANNEL);
      p.connection.sendPacket(new PacketPlayOutServerDifficulty().set("String_Node_Str",p.getWorld().difficulty()));
      p.connection.sendPacket(new PacketPlayOutSpawnPosition().set("String_Node_Str",p.getSpawnLocation()));
      p.connection.sendPacket(p.abilities.asPacket());
      p.connection.sendPacket(new PacketPlayOutPlayerCompleteMove().set("String_Node_Str",p.getSpawnLocation()).set("String_Node_Str",(byte)0));
      p.sendChunks(3);
      p.connection.sendPacket(PacketPlayOutStatistics.DEFAULT_STATISTIC);
      Slot[] slots=new Slot[44];
      slots[43]=new Slot(new Item(Substance.APPLE));
      for (      Entity entity : p.getWorld().entities()) {
      }
      ThreadsHandler.playerExecutor().assign(p);
    }
  }
);
  return p;
}

Fixed version (ground truth):
public static Player spawnPlayer(ClientConnection connection,UUID id){
  CompoundTag offlinePlayer=(OfflinePlayer.getOfflinePlayer(id) == null) ? null : OfflinePlayer.getOfflinePlayer(id).asNbt();
  if (offlinePlayer == null) {
    offlinePlayer=OfflinePlayer.generatePlayer(id);
  }
  final TridentPlayer p=EntityBuilder.create().uuid(id).spawnLocation(TridentServer.WORLD.spawnLocation()).executor(ThreadsHandler.playerExecutor()).build(TridentPlayer.class,ParameterValue.from(CompoundTag.class,offlinePlayer),ParameterValue.from(TridentWorld.class,TridentServer.WORLD),ParameterValue.from(ClientConnection.class,connection));
  p.executor.execute(new Runnable(){
    @Override public void run(){
      p.connection.sendPacket(new PacketPlayOutJoinGame().set("String_Node_Str",p.getId()).set("String_Node_Str",p.getGameMode()).set("String_Node_Str",p.getWorld().dimension()).set("String_Node_Str",p.getWorld().difficulty()).set("String_Node_Str",(short)10).set("String_Node_Str",LevelType.DEFAULT));
      p.connection.sendPacket(PacketPlayOutPluginMessage.VANILLA_CHANNEL);
      p.connection.sendPacket(new PacketPlayOutServerDifficulty().set("String_Node_Str",p.getWorld().difficulty()));
      p.connection.sendPacket(new PacketPlayOutSpawnPosition().set("String_Node_Str",p.getSpawnLocation()));
      p.connection.sendPacket(p.abilities.asPacket());
      p.connection.sendPacket(new PacketPlayOutPlayerCompleteMove().set("String_Node_Str",p.getSpawnLocation()).set("String_Node_Str",(byte)0));
      p.sendChunks(3);
      p.connection.sendPacket(PacketPlayOutStatistics.DEFAULT_STATISTIC);
      Slot[] slots=new Slot[44];
      slots[43]=new Slot(new Item(Substance.APPLE));
      p.connection.sendPacket(new PacketPlayOutWindowItems().set("String_Node_Str",0).set("String_Node_Str",slots));
      for (      Entity entity : p.getWorld().entities()) {
      }
    }
  }
);
  return p;
}