So i found a solution for the too bright image. This is not an AR bug, but with every new project in 4.25. It affects camera footage and rendered actors, also in non-AR projects.
Reason is a new workflow for Auto Exposure in UE4.25: Trello
No default template accounts for this change yet, and so everything is 1 stop over-exposed.
To fix this one has to go to the rendering settings and set Auto Exposure Bias to 0 (default was 1).
Mobile HDR still needs to be enabled because of darkness-bug.
Then it looked good so far, but a shader used for people occlusion / person segmentation was not compiled for some reason.
An inexplicable error in packaging hinders further testing for now…
Cheers!