While working on Steam network connectivity via Tryibion's Steamworks plugin, I discovered a problem with how the driver is working within Flax. It seems the implemented method for sending a message to multiple targets doesn't get invoked.

Peer code:

var message = localNetworkPeer.BeginSendMessage();
localNetworkPeer.EndSendMessage(channelType, message, networkConnections); // networkConnections is NetworkConnection[]

Driver code that should be invoked, but is not:
https://github.com/Tryibion/FlaxFacepunchSteamworks/blob/c21b9e9e07c411b4dff25560283f98beb9907130/Source/FacepunchSteamworks/FacepunchNetworkDriver.cs#L222

Along the way trying to diagnose the problem, I added the following code to attempt to hijack the call and discover how the parameters were being seen by putting this method in the driver:

public void SendMessage(NetworkChannelType channelType, NetworkMessage message, object targets)
{
    Debug.Log($"[!!!] Targets type: {targets.GetType()}");
}

// result:
[ 00:06:38.217 ]: [Info] [!!!] Targets type: FlaxEngine.Interop.ManagedArray

It seems the code that prepares the NetworkConnection[] for this variant of SendMessage() has a bug, and thus the desired method doesn't get invoked.

Maybe worth noting: the other SendMessage() variants seem to be working properly.
Also worth noting, if I call the SendMessage() method with multiple targets directly on the driver, it works fine; it only fails when called through the NetworkPeer.