First Remote Client Fails for KCP + IL2CPP Builds

[MrGadget1024]

Describe the bug
Host client is fine. First remote client connects but hangs for 10 seconds and disconnects.
Telepathy also works correctly - bug only shows for KCP transport.

[IMPORTANT] How can we reproduce the issue, step by step:

• Build Basic example as Server Build with IL2CPP selected in Project Settings
• Enter play mode in editor, click Client
• Client never shows player object, disconnects in 10 seconds
• Click Client again...client works normally on 2nd and subsequent attempts

Note that console shows KCP: received unreliable message in state Connected. Disconnecting the connection. 5 times before actually kicking the client (Ping messages every 2 seconds).

Expected behavior
First client to connect should work normally

Desktop (please complete the following information):

• OS: Windows
• Build target: standalone IL2CPP
• Unity version: 2019 LTS, 2020 LTS, 2021 LTS all exhibit the bug
• Mirror branch: master

Headless Server Console Output for single client

Initialize engine version: 2019.4.30f1 (e8c891080a1f)
[Subsystems] Discovering subsystems at path D:/Unity/Builds/Mirror2019-3/Mirror2019-3_Data/UnitySubsystems
Forcing GfxDevice: Null
GfxDevice: creating device client; threaded=0
NullGfxDevice:
    Version:  NULL 1.0 [1.0]
    Renderer: Null Device
    Vendor:   Unity Technologies
Microsoft Media Foundation video decoding to texture disabled: graphics device is Null, only Direct3D 11 and Direct3D 12 (only on desktop) are supported for hardware-accelerated video decoding.
UnloadTime: 0.401800 ms
KcpTransport initialized!
UnityEngine.Logger:Log(LogType, Object)
UnityEngine.Debug:Log(Object)

(Filename: C:\buildslave\unity\build\Runtime/Export/Debug/Debug.bindings.h Line: 39)

Mirror | mirror-networking.com | discord.gg/N9QVxbM
UnityEngine.Logger:Log(LogType, Object)
UnityEngine.Debug:Log(Object)
Mirror.NetworkManager:Awake()

(Filename: C:\buildslave\unity\build\Runtime/Export/Debug/Debug.bindings.h Line: 39)

KCP: received unreliable message in state Connected. Disconnecting the connection.
UnityEngine.Logger:Log(LogType, Object)
UnityEngine.Debug:LogWarning(Object)
System.Action`1:Invoke(T)
kcp2k.KcpConnection:RawInput(Byte[], Int32)
kcp2k.KcpServer:TickIncoming()
Mirror.NetworkLoop:NetworkEarlyUpdate()
UnityEngine.LowLevel.UpdateFunction:Invoke()

(Filename: C:\buildslave\unity\build\Runtime/Export/Debug/Debug.bindings.h Line: 39)

KCP: received unreliable message in state Connected. Disconnecting the connection.
UnityEngine.Logger:Log(LogType, Object)
UnityEngine.Debug:LogWarning(Object)
System.Action`1:Invoke(T)
kcp2k.KcpConnection:RawInput(Byte[], Int32)
kcp2k.KcpServer:TickIncoming()
Mirror.NetworkLoop:NetworkEarlyUpdate()
UnityEngine.LowLevel.UpdateFunction:Invoke()

(Filename: C:\buildslave\unity\build\Runtime/Export/Debug/Debug.bindings.h Line: 39)

KCP: received unreliable message in state Connected. Disconnecting the connection.
UnityEngine.Logger:Log(LogType, Object)
UnityEngine.Debug:LogWarning(Object)
System.Action`1:Invoke(T)
kcp2k.KcpConnection:RawInput(Byte[], Int32)
kcp2k.KcpServer:TickIncoming()
Mirror.NetworkLoop:NetworkEarlyUpdate()
UnityEngine.LowLevel.UpdateFunction:Invoke()

(Filename: C:\buildslave\unity\build\Runtime/Export/Debug/Debug.bindings.h Line: 39)

KCP: received unreliable message in state Connected. Disconnecting the connection.
UnityEngine.Logger:Log(LogType, Object)
UnityEngine.Debug:LogWarning(Object)
System.Action`1:Invoke(T)
kcp2k.KcpConnection:RawInput(Byte[], Int32)
kcp2k.KcpServer:TickIncoming()
Mirror.NetworkLoop:NetworkEarlyUpdate()
UnityEngine.LowLevel.UpdateFunction:Invoke()

(Filename: C:\buildslave\unity\build\Runtime/Export/Debug/Debug.bindings.h Line: 39)

KCP: received unreliable message in state Connected. Disconnecting the connection.
UnityEngine.Logger:Log(LogType, Object)
UnityEngine.Debug:LogWarning(Object)
System.Action`1:Invoke(T)
kcp2k.KcpConnection:RawInput(Byte[], Int32)
kcp2k.KcpServer:TickIncoming()
Mirror.NetworkLoop:NetworkEarlyUpdate()
UnityEngine.LowLevel.UpdateFunction:Invoke()

(Filename: C:\buildslave\unity\build\Runtime/Export/Debug/Debug.bindings.h Line: 39)

KCP: Connection timed out after not receiving any message for 10000ms. Disconnecting.
UnityEngine.Logger:Log(LogType, Object)
UnityEngine.Debug:LogWarning(Object)
System.Action`1:Invoke(T)
kcp2k.KcpConnection:HandleTimeout(UInt32)
kcp2k.KcpConnection:TickIncoming_Authenticated(UInt32)
kcp2k.KcpConnection:TickIncoming()
kcp2k.KcpServer:TickIncoming()
Mirror.NetworkLoop:NetworkEarlyUpdate()
UnityEngine.LowLevel.UpdateFunction:Invoke()

(Filename: C:\buildslave\unity\build\Runtime/Export/Debug/Debug.bindings.h Line: 39)

[MrGadget1024]

closing as duplicate of #2900