I'm annoyed several times a day by keyboard invocation of the modern browser. I have the habit of typing [window logo], 'e', 'x', [enter] to bring up the modern browser.
After the upgrade I get the desktop version more often than the modern version. I think the modern version is used after it's recently opened. But it's unpredictable for me. I only slightly prefer the modern browser, but I do like consistent results... Before the upgrade - invocation from the search box would always bring up the modern UI.
It's something I think must be configurable: then after I waste more time looking than I expected, I give up worrying about it again for the day...