Defect code:

token = _singleWriterMutex.TryWait(options: WaitOptions.NoDelay);
if (!token.Success)
{
   if (PushToBacklog(message, onlyIfExists: !ALWAYS_USE_BACKLOG_IF_CANNOT_GET_SYNC_LOCK))
      return new ValueTask<WriteResult>(WriteResult.Success); // queued counts as success
   ...
}
...
if (!flush.IsCompletedSuccessfully)
{
  ...
  return CompleteWriteAndReleaseLockAsync(token, flush, message, lockTaken);
}

Scenario:
Thread A:

await subscriber.PublishAsync("channel-1", 1);

Thread B:

await Task.WhenAll(subscriber.PublishAsync("channel-2", 2), subscriber.PublishAsync("channel-2", 3));

1. Thread A try to publish message 1
2. Thread A takes the lock(_singleWriterMutex) first.
3. Thread B try to publish message 2
4. Thread B cannot take the lock, send to backlog.
5. Thread B create the backlog, and start the background processing.
6. Thread B returns, since queued counts as success.
7. Thread A finish the write message 1, release the lock.
8. Thread B try to publish message 3
9. Thread B takes the lock.
10. Thread B finish the write message 3, release the lock.
11. The backlog takes the lock.
12. The backlog write message 2.

Then, finally we get following result:
Channel-1: 1
Channel-2: 3, 2