SyncVars fail to initialize correctly on host after multiple objects are spawned.

[Roshinator]

Describe the bug
On a host, instantiate a prefab, spawn it, then assign the value of a non-primitive type syncvar on the spawned object. If you loop this process a few times, the syncvar starts losing the assigned value on new objects.

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1. Use the MirrorBasic example as a starting point.
2. Create a simple cube object in the scene, assign it a network identity and attach this script to it:

using UnityEngine;
using Mirror;

public class CubeTest : NetworkBehaviour
{
    [SyncVar(hook = nameof(UpdateHook))]
    public string testValue;

    public void UpdateHook(string oldI, string newI)
    {
        Debug.Log($"Old: {oldI}, New: {newI}");
    }
}

3. Save this object as a prefab to assets, remove it from the scene, and add it to the network manager's registered spawnable prefabs.
4. Create an empty scene object and assign to it a network identity and this script:

using Mirror;
using UnityEngine;

public class SpawnerTest : NetworkBehaviour
{
    public GameObject prefab;
    int repeatCount = 10;
    public override void OnStartServer()
    {
        base.OnStartServer();
        repeatCount = 10;
    }

    void Update()
    {
        if (repeatCount > 0)
        {
            GameObject g = Instantiate(prefab);
            NetworkServer.Spawn(g);
            g.GetComponent<CubeTest>().testValue = "test string";

            repeatCount--;
        }
    }
}

5. Assign the cube prefab to the SpawnerTest component in the editor.
6. Run the scene and click host.
7. Observe the console output for the UpdateHook's log statements. After the first few objects are spawned, the output should change. Example output from my system is below.
8. If similar output isnt observed, maybe try increasing the repeatCount.

Expected behavior
A clear and concise description of what you expected to happen.
All of these logs should consistently show "test string" as the value of the syncvar for both the old and new parameters.
CORRECTION: All of these logs should show blank for the old value and "test string" for the new value.

Screenshots
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Desktop (please complete the following information):

• OS: Windows
• Build target: Windows (observed in editor)
• Unity version: 6000.3.10f1
• Mirror branch: 96.9.19

Repro project, open the mirror/basic/scenes/MirrorBasic.unity scene.
Mirror 96.9.19 Bug.zip

Additional context
Add any other context about the problem here.

[JesusLuvsYooh]

If you switch your string sync var change to before the network spawn, it works btw.
g.GetComponent().testValue = "test string";
NetworkServer.Spawn(g);

Usually you want to set the sync variable before network spawning, does doing it this way solve your case?
Otherwise yes, I get same behaviour.

(or optionally, set via the objects script which triggers once its spawned in)

[MrGadget1024]

All of these logs should consistently show "test string" as the value of the syncvar for both the old and new parameters.

Incorrect...they should all have nothing for old and "test string" for new value

[Roshinator]

Small update, I observed the same effect on in-scene objects that get their syncvar values assigned to them by other in-scene objects in OnStartServer. This doesn't happen for self assigning where an object assigns its own syncvar a value in its own OnStartServer function. However even in that case it still shows the assigned parameter as both the old and new value in the hook function.