I tried looking here for any suggestions, but came up empty. Glad we only tested iOS 12 on a couple of our iPads.
If you use Guided Access, you'll want to check this:
Guided Access disable screen sleep
I can also confirm this issue still occurs with iOS 12.0.1. Current workaround is to turn off Guided Access (or use Single App Mode).
Teem and Envoy support have also confirmed the issue.
UPDATE - looks like iOS 12.1 won't fix it - will confirm tomorrow I guess...
https://www.reddit.com/r/iOS12/comments/9ha20y/guided_access_forces_screen_to_autolock/
