Why am I getting this result with my attempt to implement a basic spin-lock in C#?

Attempt:

public interface ISemaphore
{
    void Aquire();

    void Release();
}

public class SpinlockSemaphore : ISemaphore
{
    private int _cur;
    private const int B = 1;
    private const int A = 0;

    public SpinlockSemaphore()
    {
        this._cur = A;
    }

    public void Aquire()
    {
        // Locally named variables for clarity. Could obviously be
        // condensed to something like:
        // while (Interlocked.CompareExchange(ref this._cur, B, A) != A);

        var flipped = false;
        do
        {
            // Flip to B only if it is A
            var prev = Interlocked.CompareExchange(ref this._cur, B, A);
            if (prev == A)
            {
                // If here, flipped from A to B
                flipped = true;
            }
            // If here, didn't flip because it was already B
        } while (!flipped);
    }

    public void Release()
    {
        // Set to A
        this._cur = A;
    }
}

Test (driver):

var r = new Random();
ISemaphore s = new SpinlockSemaphore();
var t_base = DateTime.Now;

var tasks = Enumerable.Range(0, 26)
    .Select(i => Task.Run(() =>
    {
        var task = (char)((int)'A' + i);
        s.Aquire();
        Console.WriteLine($"I acquired the semaphore! (Task {task} at {(DateTime.Now - t_base).TotalMilliseconds} ms)");
        Thread.Sleep(r.Next() % 1000); // Simulate work
        s.Release();
        Console.WriteLine($"I released the semaphore! (Task {task} at {(DateTime.Now - t_base).TotalMilliseconds} ms)");
    }))
    .ToArray();

Task.WaitAll(tasks); // Run all tasks (concurrently)

Console.ReadKey(); // Keep Console window from closing

Output: examples of acquisitions before releases

I acquired the semaphore! (Task B at 88.7629 ms)
I released the semaphore! (Task B at 635.2041 ms)
I acquired the semaphore! (Task C at 635.2041 ms)
I released the semaphore! (Task C at 906.7672 ms)
I acquired the semaphore! (Task D at 906.7672 ms)
I released the semaphore! (Task D at 1427.5738 ms)
I acquired the semaphore! (Task E at 1427.5738 ms)
I released the semaphore! (Task E at 1565.426 ms)
I acquired the semaphore! (Task A at 1565.426 ms)
I released the semaphore! (Task A at 2387.7001 ms)
I acquired the semaphore! (Task H at 2387.7001 ms)
I released the semaphore! (Task H at 3265.8058 ms)
I acquired the semaphore! (Task I at 3265.8058 ms)
I released the semaphore! (Task I at 3586.6978 ms)
I acquired the semaphore! (Task J at 3586.6978 ms)
I released the semaphore! (Task J at 3729.4946 ms)
I acquired the semaphore! (Task G at 3729.4946 ms)
I released the semaphore! (Task G at 4720.5139 ms)
I acquired the semaphore! (Task F at 4720.5139 ms)
I released the semaphore! (Task F at 5077.1144 ms)
I acquired the semaphore! (Task K at 5077.1144 ms)
I acquired the semaphore! (Task L at 5782.0138 ms)
I released the semaphore! (Task K at 5782.0138 ms)
I released the semaphore! (Task L at 6746.3076 ms)
I acquired the semaphore! (Task N at 6746.3076 ms)
I released the semaphore! (Task N at 7412.1375 ms)
I acquired the semaphore! (Task O at 7412.1375 ms)
I released the semaphore! (Task O at 8137.582 ms)
I acquired the semaphore! (Task M at 8137.582 ms)
I released the semaphore! (Task M at 8327.2192 ms)
I acquired the semaphore! (Task R at 8327.2192 ms)
I released the semaphore! (Task R at 8907.7793 ms)
I acquired the semaphore! (Task S at 8907.7793 ms)
I released the semaphore! (Task S at 9445.4624 ms)
I acquired the semaphore! (Task P at 9445.4624 ms)
I released the semaphore! (Task P at 10231.1811 ms)
I acquired the semaphore! (Task Q at 10231.1811 ms)
I released the semaphore! (Task Q at 10368.8125 ms)
I acquired the semaphore! (Task V at 10368.8125 ms)
I released the semaphore! (Task V at 10992.4072 ms)
I acquired the semaphore! (Task U at 10992.4072 ms)
I released the semaphore! (Task U at 12019.8456 ms)
I acquired the semaphore! (Task T at 12019.8456 ms)
I released the semaphore! (Task T at 12303.2822 ms)
I acquired the semaphore! (Task Y at 12303.2822 ms)
I released the semaphore! (Task Y at 12378.0044 ms)
I acquired the semaphore! (Task X at 12378.0044 ms)
I acquired the semaphore! (Task W at 12705.6469 ms)
I released the semaphore! (Task X at 12705.6469 ms)
I acquired the semaphore! (Task Z at 13671.7837 ms)
I released the semaphore! (Task W at 13671.7837 ms)
I released the semaphore! (Task Z at 14262.6353 ms)

So far I've been unable to figure out the issue through Debugging and re-thinking the interleavings to convince myself it should work.

Ways I've tried to solve it through brute force:

• Change _cur to have modifier volatile
• Litter the code with Thread.MemoryBarrier()s

I'm anxious to hear what obvious flaw I'm overlooking :)