Don’t know if you’re still needing help with this, but I just ran into it.
On Gnome, it would split out once you closed and reopened the app, but kde appears to auto-group things.
I found that, if you right click on the web app (in my case, it was Google Messages) in the Task Manager/Bar, go to More, and uncheck the “Allow this to be grouped” then it would split Messages out from it’s parent browser app.
Note: to click on Messages, I had to mouse over Brave, then right click on the preview of Messages to get to it. Brave itself is still grouped so new windows it generates will group correctly.