RyuJIT C# wrong sum result with /optimize

Question

I've this piece of code:

private void AnswerToCe(int currentBlock, int totalBlock = 0)
{
    byte[] bufferToSend;
    byte[] macDst = mac;
    byte[] macSrc = ConnectionManager.SInstance.GetMyMAC();
    byte[] ethType;
    byte[] header;

    if (Function == FlashFunction.UPLOAD_APPL || Function == FlashFunction.UPLOAD_BITSTREAM)
    {
        ethType = BitConverter.GetBytes((ushort)EthType.ETH_TYPE_UPLOAD);
        ethType = new byte[] { ethType[1], ethType[0] };
        header = Header.GetBytes((ushort)binaryBlocks.Count, (ushort)(currentBlock + 1), (ushort)binaryBlocks[currentBlock].Length);
        int index = 0;
        bufferToSend = new byte[macDst.Length + macSrc.Length + ethType.Length + header.Length + binaryBlocks[currentBlock].Length];
        Array.Copy(macDst, 0, bufferToSend, index, macDst.Length);
        index += macDst.Length;
        Array.Copy(macSrc, 0, bufferToSend, index, macSrc.Length);
        index += macSrc.Length;
        Logger.SInstance.Write(index.ToString(), "test index pre");
        Array.Copy(ethType, 0, bufferToSend, index, ethType.Length);
        index += ethType.Length;
        Logger.SInstance.Write(index.ToString(), "test index post");
        Array.Copy(header, 0, bufferToSend, index, header.Length);
        index += header.Length;
        Array.Copy(binaryBlocks[currentBlock], 0, bufferToSend, index, binaryBlocks[currentBlock].Length);
    }

If I build my application in Debug mode everything is ok, test index pre prints 12 and test index post prints 14. the same in Release mode with Optimize code unchecked. if i test with Optimize code checked test index post prints 18 instead of 14.
Same result if I replace index += ethType.Length; with index += 2;. seems only index++;index++; is working.
I tried this code in an empty application and sums are ok.
App is multithreading but there isn't no concurrency here.
Decompiled code from DLL seems ok.
Any ideas why this happen?

EDIT: Happens only when app is compiled for x64. x86 is ok.
EDIT 3: some info of the build env:
visual studio 15.0.0-RTW+26228.4
framework 4.7.02053
can trigger this issue on framework 4.6.2 and 4.7. other frameworks aren't tested.
EDIT 5: new, smaller example project. no dependencies needed.
EDIT 6: disassembly of the test project here. (too long to post it here)

Answer 1

It was an already reported bug in RyuJIT, more details here. Will be fixed in an hotfix soon.