How to unlock your iPad using an app
Apple today released iOS 11.3.1 for iPad, a minor update that includes the following fixes: Lock screen widget in apps that include a lock screen will now open to reveal the app instead of a new lock screen.
Improved support for locking your iPhone with a third-party app.
Improvements to the lock screen widget to better indicate that you’re unlocking with the Apple Watch or the lock icon on your lock screen on iPad.
Fixed a bug where iOS 11 was failing to detect that your device was locked.
Updated Apple Watch and iPhone support for the new Apple Watch Series 3.
If you’re having trouble accessing apps or syncing, try the following instructions: Open Settings.
Go to General > iCloud.
Under iCloud, check the box next to ‘Restore last used iCloud account’ to restore your iCloud account.
In Settings, click the lock tab at the bottom of the screen and select ‘Back up all settings.’
Then, click Restore settings.
That should fix the issue.
The fix for the issue will be applied to iOS 11, Apple says, and is available in the next update.