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...
I've also had this issue with web-clips and other applications. Tested on iOS 12.1 and no resolution for us either. From what I can find it seems like the isIdleTimerDisable value is being ignored with guided access.
Fingers crossed for some solution soon! If I find anything I will post it here :)
FIXED! Got this from following the incident status on Teem:
Potential iOS 12 and EventBoard issue
Apple has included an update in iOS 12.1.1 which allows customers to use Guided Access Mode and EventBoard again! EventBoard users will need to update affected devices to v12.1.1 change the Auto-Lock setting. (Settings -> General -> Accessibility -> Guided Access -> Mirror Display Auto-Lock = ON)
My initial testing's been good - that auto-lock setting got me at first (I thought simply upgrading would solve). But once I made the setting change, it's been solid.