With a freshly opened Sandbox and only one tab open, Ctrl + Shift + t does nothing for me.
If a different note is opened in a new tab and closed, Ctrl + Shift + t brings the closed tab back as expected. Ctrl + Shift + t is the default hotkey for the “Undo close tab” command.
These are the ways I can think of for opening a note in new tab group (there may be more):
With your cursor on a link: Ctrl + Alt + Enter
When clicking (mouse/trackpad) in the Files list or on an internal link: Ctrl + Alt + Click
Ctrl + O → choose an existing note or type name of new note → Ctrl + Alt + Enter
(not a hotkey) Context menu on file → “Open to the right” / Context menu on tab title or tab title bar three dots → “Split Right”