EssentialsPlayerListener.java

    private final Map<UUID, AbstractMap.SimpleEntry<Integer, Boolean>> worldTeleports = new HashMap<>();
    @EventHandler
    public void onPlayerChangeWorld(PlayerChangedWorldEvent e) {
        final AbstractMap.SimpleEntry<Integer, Boolean> entry = worldTeleports.remove(e.getPlayer().getUniqueId());
        if (e.getPlayer().hasPermission("essentials.fly")) {
            if (entry != null && Bukkit.getCurrentTick() == entry.getKey()) {
                e.getPlayer().setAllowFlight(true);
                if (entry.getValue()) {
                    e.getPlayer().setFlying(true);
                }
            }
        }

    }

    @EventHandler(priority = EventPriority.HIGH, ignoreCancelled = true)
    public void onPlayerTeleport(final PlayerTeleportEvent event) {
        if (event.getFrom().getWorld() != event.getTo().getWorld()) {
            if (event.getPlayer().isFlying() || event.getPlayer().getAllowFlight()) {
                worldTeleports.put(event.getPlayer().getUniqueId(), new AbstractMap.SimpleEntry<>(Bukkit.getCurrentTick(), event.getPlayer().getAllowFlight()));
                boolean isFlying = event.getPlayer().isFlying();
                Bukkit.getScheduler().runTaskLater(ess, () -> {
                    if (event.getPlayer().hasPermission("essentials.fly")) {
                        event.getPlayer().setAllowFlight(true);
                        if (isFlying) {
                            event.getPlayer().setFlying(true);
                        }
                    }
                }, 5);
            }
        }
    }