At least on my chromium variant each private window (not tab: window) gets its own isolated environment.

I frequently use this to log into websites with different user accounts for testing purposes.

Good to know, I'll take a look!

To be a little more explicit, I think you need to spawn a private window from a non-private window to get a new environment. If you do it from another private window it uses the same sandbox.

And unlike private browsing windows, they persist across browser restarts until all their tabs are closed.