Ray Traced Shadows With World Position Offset on Instanced Static Mesh or Foliage

Unlikely if you are using any beta/alpha feature at all.

Lumen/nanite etc. Nothing in 5.1 would proabbly allow for this (particularly since something like 5.2 or 5.3 was released?)

The checkbox for the grass assets WPO should theorethically be either on the mesh itslef, or on the grass definition (where you add the mesh and set the density/distance for it).