Issue is present on both Unity 2018.3.6f1 & Unity 2018.3.11f1 (latest) however I can only seem to reproduce this on my laptop, as everything works fine on my PC.
I have tried both internal and external webcams but the issue is still the same. The steps I took were switching platform to android, removing the Main Camera, adding in the WikitudeCamera, changing he webcam to my external webcam and finally playing live preview.
Thank you for reporting this issue. Can you please confirm that the same webcam is working correctly on your PC, but not on your laptop? We've seen similar reports from other customers, but we're having trouble reproducing this issue internally, so every bit of information help.
Additionally, can you try changing the resolution of the Game panel, from "Free Aspect" to something else? Does matching the resolution of the camera, for example, help with the issue?