Subsurface bug?

can anyone recognise this issue?
The back of the leaves are almost pitch black. no subsurface, no ambient light illuminates the back of the leaves.

I’m stuck.

This video/pic is straight out of the box project. no change no tweaks, nothing.
it is the ‘Electric Dreams’ environment from epic.

1- It is fine when sun is hiiting them directly. it is not fine when the plants are in shades (affected by GI, Skylight)

2- This happened mid working on a lighting scenario.

3- traced back my steps, reset everything, and nothing changed.

4- Created the new project and the problem is still there.

5- Verified the Engine version, just in case, and it still is there.