When you install a site as an app/pwa - if you close the app with the red X and then click the icon again to open it no window generates, you have to click the icon and then file > new window
Only works if you force quit (cmd + q) the app on close and then re-open!
Tested with YouTube, Zendesk, Jira all installed as an app.
It would open up another window if none already open.
For other apps on MacOS such as Outlook/OneNote etc - if you click on the icon and no windows open but the little indicator dot meaning it is active is present, it just opens a new window!
Version 0.99.132.2-rc (WebKit 621.1.2.111.4)
Sequoia (15)