Upgrading requires running update.php which updates the database. But here is where I was fooled.
You usually put the site in Maintenance mode. Install your new stuff. The site still continues to show the Maintenance Mode default page. Then you would look at a Report of the Status and that would tell you if the database needed update (at least half the time it does not). Then you update the database if needed.
But this time when I installed 8.4.0 that installation took down the site completely including the Maintenance Mode default. Everything appeared broken. [I have never seen that before during an update of a module, theme, or core.] Eventually I tried update.php. Apparently 19 different updates to the database were needed. It did them and everything seems good.
So, part D’Oh from me, but also part one does not expect the Maintenance Mode page to disappear.
[Hope this helps someone.]