The recent iOS 12.3.1 Update affects the external display. On the new iPad Pros with USB-C the display freezes and is not useable. On older devices it works, but you cannot reconnect an external display once the session is up and running.
Using Airplay is not affected by the update - this seems to work.
iOS 12.0.1 seems to be stable.
We are looking into the problems and are working on a fix.
If you need the external display - you better do not update your device.