Is it possible to get non-zero length array with only-zeroes bytes during Aes encryption? [closed]

Question

Several users have encountered strange issues with my app/game. They reported that their save files have become invalid. I've tried to investigate the problem and haven't found anything.

Here is a pipeline:

1. in-game data converts to json file (string)
2. json file converts UTF8 bytes
3. UTF8 bytes goes thru this encryption method (using AES)

Save file extension is .dat

This is a first game where I decided to use step #3 (save data with encryption) and I didn't found anything strange and like that in testcases and on test stands.

After I've got some save files from involved users, I saw that all of those files has valid non-zero length, but have only zero bytes.

This is an example of one of those files: only zero bytes here

Here is a code of encryption:

public static byte[] Encrypt(byte[] clearBytes, string encyptionKey)
{
    using (var algorythm = Aes.Create())
    {
        algorythm.Mode = CipherMode.CBC;
        algorythm.Padding = PaddingMode.PKCS7;
        var encryptionBytes = _encoding.GetBytes(encyptionKey);
        using (var pdb = new Rfc2898DeriveBytes(encryptionBytes, _salt, _iterations, _hashAlgorithm))
        {
            algorythm.Key = pdb.GetBytes(32);
            algorythm.IV = pdb.GetBytes(16);
            using (var encryptor = algorythm.CreateEncryptor())
            {
                using (var ms = new MemoryStream())
                {
                    using (var cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write))
                    {
                        cs.Write(clearBytes, 0, clearBytes.Length);
                    }
                    return ms.ToArray();
                }
            }
        }
    }
}

Here is a code of save file to file system:

public bool Save(string relativePath, byte[] bytes)
{
    var absolutePath = Path.Combine(_applicationDataPath, relativePath);
    var fi = new FileInfo(absolutePath);
    var di = fi.Directory;

    try
    {
        if (!di.Exists)
            di.Create();

        var exists = fi.Exists;
        using (var fs = exists ? fi.OpenWrite() : fi.Create())
        {
            fs.SetLength(0);
            fs.Seek(0, SeekOrigin.Begin);

            using (var ms = new MemoryStream(bytes))
            {
                ms.CopyTo(fs);
                ms.Flush();

                return true;
            }
        }
    }
    catch (Exception ex)
    {
        Debug.LogException(ex);
        return false;
    }
}

Everything is working good if I remove encryption, no reports with broken save files there. The problem is I can't reproduce this bug on my own computer.