Thanks! This is the best explanation I have seen so far.
This is probably also related to the reason why I am having so much troubles with procedural foliage and raytracing shadows too.
I wonder though, is this a UE4 limitation right? I mean, all modern rendering softwares can have negative displacement and raytracing together. Arnold, mantra, redshift, even mental ray.
Isn’t this something the Unreal Engine team should try to fix?