Program: 11730
Project: 11730
Buggy source:
public static TridentPlayer spawnPlayer(ClientConnection connection,UUID id,String name){
  CompoundTag playerTag=OfflinePlayer.getOfflinePlayer(id) == null ? null : OfflinePlayer.getOfflinePlayer(id).asNbt();
  if (playerTag == null) {
    playerTag=OfflinePlayer.generatePlayer(id);
  }
  TridentPlayer p=new TridentPlayer(id,playerTag,TridentServer.WORLD,connection);
  p.executor=ThreadsHandler.playerExecutor();
  ONLINE_PLAYERS.put(id,p);
  p.name=name;
  p.gameMode=GameMode.CREATIVE;
  p.executor.execute(() -> {
    p.connection.sendPacket(new PacketPlayOutJoinGame().set("String_Node_Str",p.entityId()).set("String_Node_Str",p.gameMode).set("String_Node_Str",p.world().dimension()).set("String_Node_Str",p.world().difficulty()).set("String_Node_Str",(short)Trident.config().getInt("String_Node_Str")).set("String_Node_Str",LevelType.DEFAULT));
    p.abilities.creative=1;
    p.abilities.flySpeed=0.135F;
    p.abilities.canFly=1;
    p.spawnPosition=TridentServer.WORLD.spawnPosition();
    p.connection.sendPacket(PacketPlayOutPluginMessage.VANILLA_CHANNEL);
    p.connection.sendPacket(new PacketPlayOutServerDifficulty().set("String_Node_Str",p.world().difficulty()));
    p.connection.sendPacket(new PacketPlayOutSpawnPosition().set("String_Node_Str",p.spawnLocation()));
    p.connection.sendPacket(p.abilities.asPacket());
    p.connection.sendPacket(new PacketPlayOutPlayerCompleteMove().set("String_Node_Str",p.spawnLocation()).set("String_Node_Str",(byte)0));
    sendAll(new PacketPlayOutPlayerListItem().set("String_Node_Str",0).set("String_Node_Str",new PlayerListDataBuilder[]{p.listData()}));
    List<PlayerListDataBuilder> builders=new ArrayList<>();
    players().stream().filter(player -> !player.equals(p)).forEach(player -> builders.add(((TridentPlayer)player).listData()));
    TridentLogger.get().log(p.name + "String_Node_Str");
    p.connection.sendPacket(new PacketPlayOutPlayerListItem().set("String_Node_Str",0).set("String_Node_Str",builders.stream().toArray(value -> new PlayerListDataBuilder[value])));
  }
);
  return p;
}

Fixed version (ground truth):
public static TridentPlayer spawnPlayer(ClientConnection connection,UUID id,String name){
  CompoundTag playerTag=OfflinePlayer.getOfflinePlayer(id) == null ? null : OfflinePlayer.getOfflinePlayer(id).asNbt();
  if (playerTag == null) {
    playerTag=OfflinePlayer.generatePlayer(id);
  }
  TridentPlayer p=new TridentPlayer(id,playerTag,TridentServer.WORLD,connection);
  p.executor=ThreadsHandler.playerExecutor();
  ONLINE_PLAYERS.put(id,p);
  p.name=name;
  p.gameMode=GameMode.CREATIVE;
  p.executor.execute(() -> {
    p.connection.sendPacket(new PacketPlayOutJoinGame().set("String_Node_Str",p.entityId()).set("String_Node_Str",p.gameMode).set("String_Node_Str",p.world().settings().dimension()).set("String_Node_Str",p.world().settings().difficulty()).set("String_Node_Str",(short)Trident.config().getInt("String_Node_Str")).set("String_Node_Str",LevelType.DEFAULT));
    p.abilities.creative=1;
    p.abilities.flySpeed=0.135F;
    p.abilities.canFly=1;
    p.spawnPosition=TridentServer.WORLD.spawnPosition();
    p.connection.sendPacket(PacketPlayOutPluginMessage.VANILLA_CHANNEL);
    p.connection.sendPacket(new PacketPlayOutServerDifficulty().set("String_Node_Str",p.world().settings().difficulty()));
    p.connection.sendPacket(new PacketPlayOutSpawnPosition().set("String_Node_Str",p.spawnLocation()));
    p.connection.sendPacket(p.abilities.asPacket());
    p.connection.sendPacket(new PacketPlayOutPlayerCompleteMove().set("String_Node_Str",p.spawnLocation()).set("String_Node_Str",(byte)0));
    sendAll(new PacketPlayOutPlayerListItem().set("String_Node_Str",0).set("String_Node_Str",new PlayerListDataBuilder[]{p.listData()}));
    List<PlayerListDataBuilder> builders=new ArrayList<>();
    players().stream().filter(player -> !player.equals(p)).forEach(player -> builders.add(((TridentPlayer)player).listData()));
    TridentLogger.get().log(p.name + "String_Node_Str");
    p.connection.sendPacket(new PacketPlayOutPlayerListItem().set("String_Node_Str",0).set("String_Node_Str",builders.stream().toArray(value -> new PlayerListDataBuilder[value])));
  }
);
  return p;
}