We have multiple kiosks running iOS 12 that have just become paper weights. Triple tapping the home button will not break guided Access mode. Force rebooting will return the iPad directly back into the kiosk app, where it still can't end guided access mode. They are bricked. This has happened to multiple devices starting two weeks ago. Any ideas?
Looks like your devices are in Single App Mode- different from Guided Access. This is enabled through Apple Configurator or an MDM solution. To exit, connect the device to a Mac with Apple Configurator 2 installed. Then, select the device and the Actions menu (in the menu bar) select Advanced - Stop Single App Mode.
To re-enable kiosk mode, simply follow the same process, instead selecting Start Single App Mode.
Note: If Single App Mode was started using a configuration profile or MDM, you'll need to remove the profile, which can be done by selecting Actions - Remove - Profiles...