Ambient occlusion related. See this thread: Pitch black shadows on megascans fixed! (one way to fix it) - #3 by DimplyPickle
Apparently the master material was supposed to have an AO texture input. But there aren’t any. So if you open the master material of the selected Quixel assets, you can see there is an AO slot but connected to a float value node of 0.0 by default.
Meaning the AO of the whole shaded area would be pitch black. Setting it to 1 will make the AO white, so the shaded area can receive light bounce.
