To clear up any confusion....
Working in an existing project .... save it...
Then click on the File menu
   Click on the New menu
       Create the new project where ever you want.  
 
When the new project is complete the Start Screen is there.  It's no big deal just an annoyance.
 
meh

Have you read the new changes to 2017.8 EA ?
http://forum.cakewalk.com/The-Early-Access-Program-is-Now-Live-for-the-Upcoming-Release-of-SONAR-201708-m3650954.aspx
 
""Clicking the New Project tab will now display templates and in this update we have streamlined the templates that are installed with SONAR.""
 
 If you are referring to, When a project is open clicking New project takes you to start screen, I cant say I have tried that but im sure disabling start screen in preferences should cure that.

meh
From a project
File
    New
       Create the new project.  When it is complete the Start Screen is there.
 
meh

Tried this, but I can't duplicate (assuming I understand your issue correctly - you open SONAR, the start screen appears, you type Ctrl+N, a new project opens, but the Start Screen persists or returns). Also tried closing the Start Screen first, then doing File > New.
 
However - I did find out a few other things about ctrl+N while poking around that relate to creating multiple new projects (which of course assumes that "Allow Only One Project Open at a Time" in Preferences is unchecked).
 

• If you create a new project with Ctrl+N, then create a second one with Ctrl+N without making any changes to the first project, the second one will replace the first one, not add another project.
• If you create a new project with Ctrl+N, and it has any kind of content in it, and then you create a second one with Ctrl+N, the second one will appear as another project along with the first one.
• As long as the first project you created has an kind of content in it, you can keep creating new, empty projects by typing ctrl+N.