I think it's needed to unsubscribe from events in OnDestroy to prevent memory leaks, such as:

private void OnDestroy()
{
    if (StatsManager.Instance != null)
    {
        StatsManager.Instance.OnStatsChanged -= OnStatsChanged;
    }
}

This works, except for when exiting play mode, sometimes it will log:

Some objects were not cleaned up when closing the scene. (Did you spawn new GameObjects from OnDestroy?)

I believe it's because:

• Unity destroys StatsManager first.
• Unity next destroys the other GameObject that unsubcribes from StatsManager event.
• When checking StatsManager.Instance != null, it causes StatsManager to respawn since StatsManager.Instance is null. (Internal Singleton behavior)

What's the best way to avoid this issue?