Just for completion, the two videos to show that behaviour:
First one, RTX only, a circular shadow appears out of nowhere and blocks the SSS effect, that becomes obvious at flat angles:
Second, the comparsion between this scene as a NON-RTX scene (everything works as expected/intented), then switch to RTX version, where it bugs:
This workaround also does not really help, since it deactivates every shadow, that those two sided foliage objects should cast onto the gorund or other objects (check the missing shadows of the leaves). So no good 