Program: 11618
Project: 11618
Buggy source:
@Override public void setElement(int slot,ChatComponent value){
synchronized (this.lock) {
    if (value != null) {
      if (this.elements.size() > slot && this.elements.get(slot) != null) {
        this.elements.get(slot).setDisplayName(value);
        PlayOutTabListItem.PlayOutTabListItemUpdateDisplayName packet=PlayOutTabListItem.updatePlayerPacket();
        packet.update(this.elements.get(slot).getUuid(),value);
        this.getUserList().forEach(player -> ((TridentPlayer)player).net().sendPacket(packet));
      }
 else {
        List<TabListElement> addedElements=new ArrayList<>();
        for (int i=0; i < slot; i++) {
          if (this.elements.size() == i || this.elements.get(i) == null) {
            TabListElement blank=new TabListElement();
            blank.setName(this.getName(i));
            blank.setBlank(true);
            blank.setDisplayName(ChatComponent.empty());
            this.elements.add(i,blank);
            addedElements.add(blank);
          }
        }
        TabListElement element=new TabListElement();
        element.setDisplayName(value);
        element.setName(this.getName(slot));
        this.elements.add(slot,element);
        addedElements.add(element);
        if (!addedElements.isEmpty()) {
          PlayOutTabListItem.PlayOutTabListItemAddPlayer packet=PlayOutTabListItem.addPlayerPacket();
          addedElements.forEach(e -> packet.addPlayer(e.getUuid(),e.getName(),e.getGameMode(),e.getPing(),e.getDisplayName()));
          this.getUserList().forEach(player -> ((TridentPlayer)player).net().sendPacket(packet));
        }
      }
    }
 else {
      if (this.elements.size() > slot && this.elements.get(slot) != null) {
        if (slot == this.elements.size() - 1) {
          List<TabListElement> removedElements=new ArrayList<>();
          removedElements.add(this.elements.get(slot));
          this.elements.remove(slot);
          for (int i=this.elements.size() - 1; i >= 0; i--) {
            if (this.elements.get(i).isBlank()) {
              removedElements.add(this.elements.get(i));
              this.elements.remove(i);
            }
 else {
              break;
            }
          }
          PlayOutTabListItem.PlayOutTabListItemRemovePlayer packet=PlayOutTabListItem.removePlayerPacket();
          removedElements.forEach(e -> packet.removePlayer(e.getUuid()));
          this.getUserList().forEach(player -> ((TridentPlayer)player).net().sendPacket(packet));
        }
 else {
          this.elements.get(slot).setDisplayName(ChatComponent.empty());
          this.elements.get(slot).setBlank(true);
          PlayOutTabListItem.PlayOutTabListItemUpdateDisplayName packet=PlayOutTabListItem.updatePlayerPacket();
          packet.update(this.elements.get(slot).getUuid(),ChatComponent.empty());
          this.getUserList().forEach(player -> ((TridentPlayer)player).net().sendPacket(packet));
        }
      }
    }
  }
}

Fixed version (ground truth):
@Override public void setElement(int slot,ChatComponent value){
synchronized (this.lock) {
    if (value != null) {
      if (this.elements.size() > slot && this.elements.get(slot) != null) {
        this.elements.get(slot).setDisplayName(value);
        PlayOutTabListItem.UpdateDisplayName packet=PlayOutTabListItem.updatePlayerPacket();
        packet.update(this.elements.get(slot).getUuid(),value);
        this.getUserList().forEach(player -> ((TridentPlayer)player).net().sendPacket(packet));
      }
 else {
        List<TabListElement> addedElements=new ArrayList<>();
        for (int i=0; i < slot; i++) {
          if (this.elements.size() == i || this.elements.get(i) == null) {
            TabListElement blank=new TabListElement();
            blank.setName(this.getName(i));
            blank.setBlank(true);
            blank.setDisplayName(ChatComponent.empty());
            this.elements.add(i,blank);
            addedElements.add(blank);
          }
        }
        TabListElement element=new TabListElement();
        element.setDisplayName(value);
        element.setName(this.getName(slot));
        this.elements.add(slot,element);
        addedElements.add(element);
        if (!addedElements.isEmpty()) {
          PlayOutTabListItem.AddPlayer packet=PlayOutTabListItem.addPlayerPacket();
          addedElements.forEach(e -> packet.addPlayer(e.getUuid(),e.getName(),e.getGameMode(),e.getPing(),e.getDisplayName()));
          this.getUserList().forEach(player -> ((TridentPlayer)player).net().sendPacket(packet));
        }
      }
    }
 else {
      if (this.elements.size() > slot && this.elements.get(slot) != null) {
        if (slot == this.elements.size() - 1) {
          List<TabListElement> removedElements=new ArrayList<>();
          removedElements.add(this.elements.get(slot));
          this.elements.remove(slot);
          for (int i=this.elements.size() - 1; i >= 0; i--) {
            if (this.elements.get(i).isBlank()) {
              removedElements.add(this.elements.get(i));
              this.elements.remove(i);
            }
 else {
              break;
            }
          }
          PlayOutTabListItem.RemovePlayer packet=PlayOutTabListItem.removePlayerPacket();
          removedElements.forEach(e -> packet.removePlayer(e.getUuid()));
          this.getUserList().forEach(player -> ((TridentPlayer)player).net().sendPacket(packet));
        }
 else {
          this.elements.get(slot).setDisplayName(ChatComponent.empty());
          this.elements.get(slot).setBlank(true);
          PlayOutTabListItem.UpdateDisplayName packet=PlayOutTabListItem.updatePlayerPacket();
          packet.update(this.elements.get(slot).getUuid(),ChatComponent.empty());
          this.getUserList().forEach(player -> ((TridentPlayer)player).net().sendPacket(packet));
        }
      }
    }
  }
}