You do have point.
I down loaded a Sentinel 1 image, and i did exactly what you mentioned here.
(Why did you propose Terrain Flattening prior to RDTC? why not SAR-Sim provided that you said that SAR-Sim is more accurate?)
I saw that the blackish stripes do coincide with the roads on Google Earth.
Ultimately, it should be an ALOS 2 inherited problem.
Any thoughts?