I did a few more tests on different scenarios but still getting the same results. I think I’m gonna stop on this and would just wait for RC update of having an option to change the orientation on mesh export.
Zbrush is definitely left-handed Y-up axis. Here’s a good discussion about this, including other 3d programs:
https://blender.community/c/rightclickselect/HGbbbc/
http://archive.zbrushcentral.com/showthread.php?5481-Zbrush-Coordinate-system
For RC, I would think its X-up right handed. This problem still exists for quite sometime (almost 3 yrs) and still happening today, and the way things are going right now, no one seems to know which side is the problem.
I’ll try to ask on zbrush forum, but I won’t count on it as I also tried zbrush 2018 and still got the same result. I doubt its just a normal bug. Something is probably broken in between these two apps that requires them to sit down and talk.