If you're using Reality Capture for photogrammetry, avoid using the DNG Picture format phones typically default to for RAW. It'll only use an internal lower resolution jpeg embedded in the DNG file, and not an actual high resolution RAW photo.
Strange. Here is a quick test with a POM material as described in the old unreal documentation entry posted above (Heightmap Channel - blue, reference plane - 0.5), with some default Megascans texture sets assigned: edit: ah, you're probably using Pixel Depth Offset.