This started happening with either 9.4 or 9.4.1 actually, and is by design. It is an initial step in eventually migrating to having most if not all applications begin opening in their own tab.
If you've closed Admin as a TDNext tabbed item, there will not be a way to move it back into that interface going forward.
Should you feel this is a disruptive feature you can definitely address it via a product idea ticket which can be submitted from our Client Portal Home page. Just use the "Submit a Product Idea" button to send that in. We welcome all feedback from our clients.
Please let me know if you have any additional questions about the TDAdmin functionality.