I am losing my mind. Look at this code. Using concurrent queue

public void Save(T model)
{
    _queue.Enqueue(model);

    Debug.WriteLine("Enqueue " + model.ToString()); // <-- this changes everything!!!
    
    StartProcess();
}

public void Flush()
{
    Task? t = StartProcess();
    if (t != null)
        t.Wait();
}



private Task? StartProcess()
{
    Task? t = null;
    
    if (!_isSaveActivated)
    {
        lock (_lock) // ensure only one persisting loop running
        {
            if (!_isSaveActivated)
            { 
                _isSaveActivated = true;
                
                t = Task.Run(() => ExecuteProcess());
            }
        }
    }

    return t; 

}


private void ExecuteProcess()
{
    // this will run until queue is empty
    // but lets break by 10 not to delay too much
    int count = 0;
    List<T> saveList = new List<T>();

    try
    {

        while (_queue.TryDequeue(out T? item))
        {
            count++;
            saveList.Add(item);

            // if behind picked item there is empty queue, we should save as many items as there are
            if (count == 10 || !_queue.TryPeek(out T? nextItem))
            {
                Save(saveList);
                // reset values for the next 10-item loop
                count = 0;
                saveList.Clear();
            }
        }
        
    }
    catch (Exception ex)
    {
        // trace
    }
    finally
    {
        // unblock the next potential thread
        _isSaveActivated = false;
        
    }

    void Save(IEnumerable<T> persistList) // local
    {
        _dbContextProxy.AddRange(saveList);
        _dbContextProxy.Save();

    }
}

And in the unit test, a simple loop

for (int i = 1; i <= totalCount; i++)
    persister.Save(new TestModel1() { Id = i, Desc = "Item " + i });

Assert.AreEqual(633, proxy.TototalCount, "Total count should be {0}", 633);

Here is the freak show. I have the collector, every time _dbContextProxy.Save(); runs, total processed items is added to. When this line is commented out // Debug.WriteLine("Enqueue " + model.ToString()); <-- this changes everything!!! my totals is 0 . But when I have that line uncommented - it works!! Both, RUN and Debug. And I did it like 20 times already - commented, uncommented and recompiled in between. And this behavior reproducible each time.

One thing I must add that this whole code being run from Task.Run(...) somewhere down the stack.

What do you see? Help appreciated