Steps to reproduce the issue

setup (or use) a Joomla 3.9 / 3.10 site
create nested categories e.g News > TSX Venture > NM
create article in the NM category e.g. test article
setup menu to main category News > type category blog
set article options > integration > legacy router

1] on front-end go to menu test, click the tsx venture category > click the nm category > open test article > note the URL

upgrade 3.9 to 3.10 to 4.0
you want to retain your URL for SEO purposes so make sure that you have turned OFF 'Remove ID's from URLs' in article options

2] on front-end go to menu test, click the tsx venture category > click the nm category > open test article note the URL

Expected result

both URLs should be the same

Actual result

URL 1]: http://localhost/test/310/test/12-tsx-venture/nm/7-test-article
URL 2]: http://localhost/test/310/test/11-tsx-venture/12-nm/7-test-article

Additional comments

Upgrading existing J3 sites (with legacy router) will break B\C on the URL build with J4 (modern router) with Article IDs