Repro Rate 5/5
Confirmed to be a regression. Regression tested in //UE4/Release-4.27 at CL 18319896 and it did not occur.
This was tested on a prepackaged project for compat testing. Only a black screen is shown even after allowing the app to access the camera. I tested around it by also rotating between landscape and portrait but it still doesn't appear to display the camera in the screen. Further testing also show that this is a problem that appears only in a package built with Shipping configuration since a package built with development configuration appears to work.
Steps:
Actual Result: Camera passthrough doesn't begin.
Expected Result: Camera passthrough begins.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-147930 in the post.