OK, I just ran into a novel UX scenario on the first day of rollout to our users. It is to do with the new "All Work" tab on the horizontal menu of a project.
In the past to search "all work" users would have used the "Advanced Search" or "Filters" screen which is clearly project agnostic.
With this new horizontal menu, and the project name now shown the "all work" tab, these newer users are intuiting that their query results are project specific, like the "list" tab does. This resulted in them seeing work items from other projects.
With the "search" dialoged being so visible in the new UX, I don't see a reason for this "All work" tab.
I would suggest that Admins remove this new 'all work' tab on your project's horizontal navigation menu prior to enabling the new UI. This will avoid you facing users with a similar confusion and reduce some of the cognitive load on your users through a simpler screen.
Example, showing how you can search for items not in the project but within the project.
