OpenXml Dotx to Docx Word Found Unreadable Content

I am trying to write code to fill a template's content controls then save it as a new file.

I found this very helpful entry Word OpenXml Word Found Unreadable Content And used the code there.

I copied the code from that post as shown below

   public static MemoryStream ReadAllBytesToMemoryStream(string path)
    {
        byte[] buffer = File.ReadAllBytes(path);
        var destStream = new MemoryStream(buffer.Length);
        destStream.Write(buffer, 0, buffer.Length);
        destStream.Seek(0, SeekOrigin.Begin);
        return destStream;
    }

    public static void Generate()
    {

        using MemoryStream stream = ReadAllBytesToMemoryStream(@"c:\Templates\TemplateTest.dotx");

        using (WordprocessingDocument wpd = WordprocessingDocument.Open(stream, true))
        {
            wpd.ChangeDocumentType(WordprocessingDocumentType.Document);
        }

        File.WriteAllBytes(@"c:\Templates\TemplateTestOutput.docx", stream.GetBuffer());
        return;
    }

It successfully creates the file, but the problem is, whenever I open the new docx file, it gives the "Word Found Unreadable Content" error. The template I made isn't complex, it just has 3 content controls with regular text for labels. I also tried copying a regular docx with just some lines of text, same error.

Whenever I click ok, on the Word Found Unreadable Content error, it shows the document just fine. I'm not sure what I'm doing wrong, I'm not even editing anything at this point.