Tabs Studio is a Visual Studio and SSMS extension empowering you to work comfortably with any number of open documents.
v4.7.0 adds support for integrated vertical tabs in VS 2019 v16.4. More specifically, when vertical tabs are activated in VS 2019, Tabs Studio reuses this area to show tabs, instead of showing them in a separate tool window:
See the full what’s new list. Download the installer.
Making Visual Studio perfect 
Unfortunately, there is no option to deactivate this and use the old behaviour….
You can set Visual Studio tab layout to Top and use old Tabs Studio behavior.