I'm actually familiar with the window of tolerance. The old consult-T who had diagnosed me as dissociative explained it to me, and drew me a nice picture to take home showing it! (Sigh, she would have been a great T for me... but I digress).

If you're not in the window of tolerance and you're dissociating - ending the session is not really helpful at all. She should be doing things like... slowing down and creating a safe environment, so you can tolerate being there, and working on grounding techniques and ways to help you stay safe and present.

I can't see any value in ending a session because someone is dissociating, it doesn't make sense to me? Did she say anything when she ended the session? Are you sure she ended early (versus it feeling early because you lost time?) Do you remember how the session ended? Not trying to accuse you of anything, I'm just puzzled too...