Popular app Duet Display, that turns your iPad into an extra display for your Mac via the Lightning USB cable, is not yet supported on the latest macOS 10.13.4 release. The developer has updated the app’s FAQ page to reflect this while recommending all Duet Display users to stay on macOS 10.13.3 till the issue has been resolved.
*We recommend staying on macOS 10.13.3*
Installing macOS release Version 10.13.4 will cause Duet Display stop functioning after the OS upgrade. We have alerted Apple to this issue and are working hard to find a resolution. These features continue to work as expected in 10.13.3.
The developers also recommend users to contact Apple though their bug reporting at this link, noting that it will help have the bug prioritized if Apple is aware how many people are affected by this issue.
“We are actively looking into workarounds so that duet is not affected by the macOS bug, and will keep you informed” note the developers. In case you have any questions, you may contact them directly via email.