I figured out the workaround today. I created a solution with two test projects. I created a Windows Forms project which, of course, defaulted to names Form1.cs and Form1.Designer.cs. Then before adding controls, I renamed the files to FormTest1.cs, etc. I then dragged three controls (a Button, a Label, and a TextBox) from the Toolbox to the form. I built and ran the project and, as I have seen before, the form was displayed with no controls. I closed the application and examined the FormTest1.Designer.cs file and found that there was no information about the controls I had dragged in.
I then created a second Windows Forms project with the default name of Form1.cs. This time, before renaming the form I dragged the same three controls as in the previous project to the form. Then I renamed the form file to FormTest2.cs. I built and ran the project and this time the form was displayed with all three forms. I closed the application and examined the FormTest2.Designer.cs file and found that all the information about the dragged in controls was present.
It seems Visual Studio just can't handle renaming the Form files before adding controls.
You can see the projects I created to demonstrate this on GitHub at https://github.com/nick1941/DesignerTest. Project1 is the result of renaming before adding controls. Project2 is the result of renaming after adding controls.
I have reported this problem to the Microsoft Visual Studio development team.