I would say it works like this by design. You don’t get 2 dialogues it’s just that the existing dialogue changes to still give the user the Defer option if he closes the app himself without clicking the “Close Apps & Install” button.
If you close the app(s) with the “Close Apps & Install” button it you get an “automatic” close and install experience.
As an example (to prove @JFP’s excellent explanation), if before you launch the PSADT script / deployment make sure Excel in not running, then the install should commence without the Show-ADTInstallationWelcome screen displaying
Switching to the Classic UI the installation continues without prompting for an additional confirmation. This is the behavior in 3.10.2. I can live with the classic UI, though it would be nice if the Fluent UI worked the same way as the classic UI.