Tags not being deleted in Microsoft Office Object model - interop

Question

I have this weird problem with the Powerpoint office model. For various reasons I store some user data in the tags of the presentation. But I need to be able to strip them out. The delete just fine in the object model, but when I save the presentation, the tags are restored. Here is a sample program:
// Reference to Microsoft.Office.Core and Microsoft.Office.Interop.Powerpoint
// required to execute this code.
using System;
using Ppt = Microsoft.Office.Interop.PowerPoint;
namespace PptCleaner
{
class Program
{
static void Main(string[] args)
{
var filename = args[0];
var app = new Ppt.Application();
var pres = app.Presentations.Open(filename);
// Presentation has four tags, so pres.Tags.Count == 4
while(pres.Tags.Count > 0)
pres.Tags.Delete(pres.Tags.Name(1));
// After loop pres.Tags.Count == 0
pres.Save();
// After save pres.Tags.Count has gone back to 4
pres.Close();
app.Quit();
}
}
}

Answer 1

Per OP's request this is the code with which I was unable to reproduce the issue:
using System;
using Microsoft.Office.Interop.PowerPoint;
namespace Test
{
class Program
{
static void Main(string[] args)
{
Application ppt = new Application();
Presentation pres = ppt.Presentations.Open(#"C:\test2.pptm");
Console.WriteLine("count before: " + pres.Tags.Count);
while (pres.Tags.Count > 0) pres.Tags.Delete(pres.Tags.Name(1));
Console.WriteLine("count after: " + pres.Tags.Count);
pres.Save();
pres.Close();
ppt.Quit();
Console.ReadLine();
}
}
}
I had previously added four tags to test2.pptm via a macro in the workbook. The first time I ran this program I got the output 0 followed by 4, the second time I got 0 both times (as expected).