I've been thinking about this, and i'm not entirely sure we should "fix" this. If an authenticated user can see the site while it's in maintenance mode, i'm positive a lot of people will be putting their sites in maintenance mode, and then forgetting to turn if off because thhey can just see the frontend.
Fixing this itself won't be hard (add a check for a user in src/Event/Subscriber/MaintenanceModeSubscriber.php), but i'd like to bump this for right now.. To fix this properly, we'll need to make it extra visible in both front- and backend that the site is still in Maintenance.
For now, you can use the "preview" option from the backend to view single pages, whilst maintenance is on.
Sure that is an option for now. As a recommendation I would suggest solving this need to make it visible by adding something like the "edit-this-page"-Button from Ivo's extension to the page. Like a big "MAINTENANCE" in red letters at the lower right corner or something should do the trick in my opinion.
It is in configuration notices, but not everybody will have that installed. From the v3 days, I know that a bunch of developers will actively want to remove it, because they feel they don't need it. (which is true, until they do ;-) )
Just joining with my opinion here (as I was affected by this today), a reason I would want to put a site in maintenance mode is because I want to make changes (new content, re-arrange content, apply updates, etc.) without this being visible to external users.
If I cannot see my site whilst it's in mainenance mode, there is no way I can test the site is working after an upgrade, or my content is linked and appearing correctly, without putting it live for everyone.
Also the comment in the default config.yaml is misleading compared to the current functionality 😉
# Toggle maintenance mode on or off. Note: If you're logged in, you'll still see
# the website as usual. Use an incognito window to see the "maintenance" page.
Drupal sorts the issue of the admin not realising the site is in maintenance by putting a banner on every page - whether it's in the admin, or on the front-end of the site, so there is no way you can miss it.