GameLift multiple processes can not detect open ports on Windows Server

I want to be able to run multiple processes in a GameLift fleet. When I test with gamelift locally, the following code can find the next open port on my machine (Windows). Concurrent server.exe processes will open on consecutive ports if one is in use, and are passed to the GameLift Server. In an actual GameLift fleet, however, multiple processes are trying to start on the same port (5000). I have the fleet port settings open between 5000-5500 for TCP and UDP.

public static ushort GetFirstOpenUdpPort(int startingPort, int maxNumberOfPortsToCheck)
    {
        var range = Enumerable.Range(startingPort, maxNumberOfPortsToCheck);

        var portsInUse =
            from port in range
            join used in System.Net.NetworkInformation.IPGlobalProperties.GetIPGlobalProperties().GetActiveUdpListeners()
            on port equals used.Port
            select port;

        var firstFreeUdpPortInRange = range.Except(portsInUse).FirstOrDefault();

        if (firstFreeUdpPortInRange > 0)
        {
            Debug.Log($"..Open port found at {firstFreeUdpPortInRange}");
            return (ushort)firstFreeUdpPortInRange;
        }
        else
        {
            Debug.LogError($"..No valid UDP ports available in the given range ({startingPort} to {startingPort + maxNumberOfPortsToCheck})");
            return 0;
        }

    }