I have spigot version 1.8.8

I had to modify utils.js for getPlayers() to work properly.

function getPlayersBukkit(){
var result = [];

//var players_collection = server.getOnlinePlayers();
//var players = players_collection.toArray(new org.bukkit.entity.Player[players_collection.size()]);
var players = server.getOnlinePlayers();
//for (var i = 0; i < players.size(); i++){
// result.push(players.get(i));
//}
//result.push(players[0]);
// I HAD TO CORRECT THIS CODE since size() does not exist
for (var i = 0; i < players.length; i++){
result.push(players[i]);
}
return result;
}

https://hub.spigotmc.org/javadocs/spigot/org/bukkit/Server.html#getOnlinePlayers()

I've checked this with Spigot 1.9 and Java 8. I can't reproduce this issue. Can you try upgrading to Spigot 1.9 ?

After upgrade to spigot 1.9 everything works out of the box. No bug!