I'm pretty sure it's the Liquid Mix. I have the same problem. When using the Liquid Mix in a session, closing PT and the process still in the Task Manager. When I don't use de Liquid Mix in a session the problem doesn't occure.
I have sent an email to Focusrite Support. They say it's an known issue with the Liquid Mix, but because the Liquid Mix is end-of-life (or will be soon) they don't gonna fix it!
I've tries some utilities to kill the process, but without any succes. I have to reboot or wait for 5 minutes.
Best regards,
Milko