Joystick input works fine when the user first loads up interactive. However, if they refresh, any attempt to interact with the joystick will result in an error message (and no joystick event):
Error: Error while processing method: giveInput
UnityEngine.Debug:Log(Object)
Microsoft.Mixer.InteractivityManager:Log(String, LoggingLevel) (at H:/GIT/WatchBeam/interactive-sdk-csharp/interactive-sdk-csharp/Source/Microsoft.Mixer.Shared.CSharp/InteractiveManager.cs:3361)
Microsoft.Mixer.InteractivityManager:LogError(String, Int32) (at H:/GIT/WatchBeam/interactive-sdk-csharp/interactive-sdk-csharp/Source/Microsoft.Mixer.Shared.CSharp/InteractiveManager.cs:3350)
Microsoft.Mixer.InteractivityManager:LogError(String) (at H:/GIT/WatchBeam/interactive-sdk-csharp/interactive-sdk-csharp/Source/Microsoft.Mixer.Shared.CSharp/InteractiveManager.cs:3344)
Microsoft.Mixer.InteractivityManager:ProcessMethod(JsonReader) (at H:/GIT/WatchBeam/interactive-sdk-csharp/interactive-sdk-csharp/Source/Microsoft.Mixer.Shared.CSharp/InteractiveManager.cs:1459)
Microsoft.Mixer.InteractivityManager:ProcessWebSocketMessage(String) (at H:/GIT/WatchBeam/interactive-sdk-csharp/interactive-sdk-csharp/Source/Microsoft.Mixer.Shared.CSharp/InteractiveManager.cs:1391)
Microsoft.Mixer.InteractivityManager:OnWebSocketMessage(Object, MessageEventArgs) (at H:/GIT/WatchBeam/interactive-sdk-csharp/interactive-sdk-csharp/Source/Microsoft.Mixer.Shared.CSharp/InteractiveManager.cs:1342)
WebSocketSharp.Ext:Emit(EventHandler`1, Object, MessageEventArgs)
WebSocketSharp.WebSocket:messagec(MessageEventArgs)
WebSocketSharp.WebSocket:message()
WebSocketSharp.<>c__DisplayClass179_0:<startReceiving>b__1(WebSocketFrame)
WebSocketSharp.<>c__DisplayClass83_0:<ReadFrameAsync>b__3(WebSocketFrame)
WebSocketSharp.<>c__DisplayClass77_0:<readPayloadDataAsync>b__0(Byte[])
WebSocketSharp.<>c__DisplayClass55_0:<ReadBytesAsync>b__0(IAsyncResult)
Fairly quickly the stack trace goes into DLLs, so I don't know exactly what is going on, but I haven't found any dirty workarounds so this is a somewhat pressing issue.
Another note is that for some reason different results are produced when the Mixer Editor is open. When open, all users have the same joystick (rather than each client having their own like it's supposed to work). I mention this because the joystick error doesn't appear if the editor has been opened at any point while playing, so that may produce a false positive when testing.