new windows will only attach to previously used window if terminal is already running

### Windows Terminal version

1.20.11781.0

### Windows build number

10.0.19045.3930

### Other Software

_No response_

### Steps to reproduce

Settings:

- When Terminal starts: Open windows from a previous session
- New instance behaviour: Attach to the most recently used window

### Expected Behavior

Open previously used window and attach to it as new tab.

### Actual Behavior

Opens new window and the previous one.

Reproduce the behavior using Windows Terminal 1.20.11781.0 on Windows 10.0.19045.3930 with the listed session and new-instance settings. Investigate the handling of previously used windows and terminal startup, and consider the issue complete when a new instance attaches as a tab to the restored window instead of opening another window.

Written by the indexing model from the issue text.