Visual Studio caches the project file and imported
Today I found this one out the hard way:
Editing Loaded Project Files
Visual Studio caches the content of project files and files imported by project files. If you edit a loaded project file, Visual Studio will automatically prompt you to reload the project so that the changes take effect. However if you edit a file imported by a loaded project, there will be no reload prompt and you must unload and reload the project manually to make the changes take effect.
Written on February 25, 2010

Editing Loaded Project Files