I don’t know if it’s because of the 4.18.3 hotfix, or not - but if you remove all video feed backgrounds cube that you’ve created (based on Epic/Apple’s documentation for implementing ARKit, this is what you were supposed to do) - it actually works for portrait and landscape.
So actually - you don’t need anything but a player with a camera that does hit tests. No need for fixes for color and stuff, too.
Tested on iPhone 8 with iOS 11.2.2 and UE4.18.3