I think that is by design. It's not showing up on my tracker either. Usually it won't show up as enrolled until you have started doing one of the prerequisites of the exam or attempted the exam itself. As there are no prerequisites (as of what I can see) for NPDESI, you won't enroll until you actually attempt the exam.
Thanks for the reply Tom, it's good you see the same on your tracking page.
However, per the following page, Professional or any Expert level certs are pre-request for NPDESI Design and Implementation Specialist
I'm curious if I can enroll and reserve a seat for the exam via Pearson, since tracking system shows not enrolled for NPDESI.