Recently Apple launched the public beta of iOS 11.3 and we have downloaded and tested it on an iPad Pro and Air 2 device. So far the external display is working again!
Issues are still appearing when disconnecting the AV adapter during run time of the app. Then it still crashes and you need to restart the app and connect again. Otherwise no issues found so far.
You can apply for the public beta of Apple here - you just need your Apple ID. It´s also possible to go back to the official build again if you have made a backup before.
There is no official public release date yet - only rumours about this March.