[TestFixture]
public class Fixture
{
    [Test]
    public void Crashes_on_runtime_greater_or_equal_to_NET_5()
    {
        var txt = "YWJj\nZGVm"; // abc\ndef
        var base64Bytes = Encoding.UTF8.GetBytes(txt);
        var stream = new MemoryStream(base64Bytes);
        var base64Transform = new FromBase64Transform();
        var cryptoStream = new CryptoStream(stream, base64Transform, CryptoStreamMode.Read);

        var result = new MemoryStream();
        cryptoStream.CopyTo(result);

        Console.WriteLine(Encoding.UTF8.GetString(result.ToArray()));
    }
}

System.IndexOutOfRangeException : Index was outside the bounds of the array.
   at System.Security.Cryptography.FromBase64Transform.TransformFinalBlock(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount)
   at System.Security.Cryptography.CryptoStream.ReadAsyncCore(Memory`1 buffer, CancellationToken cancellationToken, Boolean useAsync)
   at System.Security.Cryptography.CryptoStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   at System.Security.Cryptography.CryptoStream.CopyTo(Stream destination, Int32 bufferSize)
   at System.IO.Stream.CopyTo(Stream destination)
   at ClassLibrary1.Fixture.Crashes_on_runtime_greater_or_equal_to_NET_5() in E:\cm_1\drive\ClassLibrary1\Class1.cs:line 20