Open a new document, preferably using your desired default template.

Turn on the option "Give new objects most recent attributes" (Options | General tab).

Draw an ellipse or a rectangle.

Select the object, access the Fill tool, and select Flat fill. If there's a fill color you'd prefer as default, assign that fill color.

Draw another shape, and verify that it has the same flat fill. If so, you can delete that object.

Save the file as a template (File | Save Template). If you want this template to be your default, check the Use As Your Default Template box under the preview window on the right in the Save Template dialog.

Start a new file based on the template. Draw another shape and verify that the object has the flat fill.

If it's working, you should be able to delete any shapes from the template and save it as a template again.

If it's still giving you bitmap fills from that template, let us know and we'll continue to work on it. You might have to reinstall, but let's try the template route first.