FYI As also happened at iOS 11.0.0, Apple seems to have broken Guided Access in iOS 12.0 GM. Even if auto-lock is set to Never, the device goes to sleep after 20 minutes. If you use an iOS 12 device as a Simple Hub, make sure to keep Guided Access turned off for now until they fix this. Auto-Lock works properly when Guided Access is turned off. The only downside to turning it off is that system-wide modal alerts do interrupt the app. These are very rare, but they include the alert that says something like “You have added a new iOS device to your Apple ID” for example. Guided Access prevents such interruptions. So be aware of it and just keep it off, or keep your Hub device on iOS 11 for now.
I would strongly encourage everyone to report this bug to Apple for prioritization purposes. Multiple bug reports is actually how they prioritize things. They fixed it quickly in iOS 11 around 11.0.1 or 2 so hopefully they’ll get to this quickly as well.