To add to my previous post: still on Windows 10 Pro, version 1709, build 16299.19.
I created two new local accounts, TestAdmin and TestUser, made the first a member of the Administrators group only, and the second a member of the Users group only. The previous local accounts I was using existed before the upgrade to the Fall Creators update
(and survived it :) . These two new accounts were created after the update and were only used for these tests, so they had little or no chance to become corrupted.
I then did a number of tests. This showed that it makes no difference whether the accounts are new, or existed previous to the Fall update. It makes no difference whether the local accounts are local admins or just ordinary users. And it makes no difference
whether you do a shutdown, or a restart. The results were always the same. There was no account switching whilst signed in. Each time the machine was switched on, exactly one local account signed in, looked around, and then did a shutdown and switch off.
In every case, if account A does a shut down, and then the machine is switched back on again, account A's signon screen is presented - as expected. If account B then signs in, where B is not A, Task Manager/Users shows account B signed in as expected, but also
account A is there too, but disconnected. That has to be wrong, surely? Account A uses quite a bit of cpu (80% - 90%) for about half a minute, too, so it's doing something (?) but then that tails off to nothing. But A just sits there, disconnected.
If account B is a local admin, and in Task Manager/Users, B does a rightclick on the disconnected account A and signs it out, then the "someone else ..." message doesn't occur at shutdown. Of course, ordinary users can't do that.
Hope this helps.