I have

If I'm running the non-preview version (D:\wt\WindowsTerminal.exe) and I issue D:\wt\WindowsTerminal.exe nt, I get a new tab in the current window (the desired behavior).

If I'm running the preview (D:\wtp\WindowsTerminal.exe) and I issue D:\wtp\WindowsTerminal.exe nt, I get a new window/instance (not the desired behavior).

What's up with that? In the second case above, "-w 0" makes no difference.

I'm using these.

Microsoft Windows 10 Pro for Workstations
10.0.19045.6216 (2009, 22H2)
WindowsTerminal 1.22.2507.30001 (1.22.2111.0)
WindowsTerminal 1.23.2507.29002 (preview) (1.23.2102.0)