"They were under the misguided impression the shutting the machine down each night was as good a restart."
- Tell me about it. Now with fast startup, shutdown really equals "hibernate", every user in the world now has to be re-trained to understand that "reboot" is better than "shutdown+restart". You could do full shutdowns all day and not understand why the bad driver that needed to reinitialize was never actually getting reloaded, but a actual restart would have fixed it instantly.
The user profile preloading is really the "use my signin info to finish installing updates after a restart" option. They praised the feature in their insider release notes at some point, but it may not have been clear that this is the related option for what you're seeing.
shutdown.exe command line program also got some new switches to take advantage of (or disable) this new behavior.