Looking at the answer to Check music file rating with VB.NET + WinForms, the likely cause is that your tags may be getting saved as iD3v2.4 and Windows only supports iD3v2.3.
You can force TagLib# to save your tags as iD3v2.3 with the following code:
TagLib.Id3v2.Tag.DefaultVersion = 3;
TagLib.Id3v2.Tag.ForceDefaultVersion = true;