A few years ago, as I was preparing my foliage libraries for sale on the Unreal Engine Marketplace, I had issues with Ray Tracing (a relatively new feature in UE 4.2X) and the Two Sided Foliage shader, particularly the Sub-Surface parameters. I wrote some extensive blog posts about this, with examples and solutions at the time, on my old blog at Blogger.
However, based on some recent online research it appears these issues have been addressed. I point to this documentation from Epic:
It mentions that both the foliage and sub-surface properties are supported.
I hope to have a chance to test out whether this is really true in UE 5.3 very soon. I'll follow up with my findings. The only previous solution to get Sub-Surface to work in the Two-Sided Foliage was to disable Ray Tracing. It would be great if this truly is fixed.