EnvDTE80.SolutionFolder hidden flag value does not seem to be saved

Question

I have a C# program that opens an existing VS solution, creates a folder with a project and then hides it. When I run the program it works fine, and I can see the folder being created and then hidden. However, when I close the solution and reopen the solution, the folder is visible again, like the value is never saved.

Here is the code that hides the folder:

EnvDTE.Project solFolder = solution2.AddSolutionFolder(projectName + "Folder");
EnvDTE80.SolutionFolder solutionFolder = (EnvDTE80.SolutionFolder)solFolder.Object;
solutionFolder.AddFromTemplate(projectTemplateFileFullName, projectFolder, projectName);
solutionFolder.Hidden = true;

When I debug the program and let it open the solution, I check again for the solutionFolder.Hidden flag and it is false. Tested on VS2019 and VS2017.

Thank you!