Hi,
this could be due to a variety of reasons and might be mitigated by changing some console variables depending on the light source (for example lowering the value of r.Shadow.Virtual.ResolutionLodBiasDirectional by -1 doubles the resolution of shadows with the associated performance tradeoffs as described here).
A few questions to help diagnose this:
Does this sudden decrease in shadow quality only happen with Nanite trees or also other (non-foliage) Nanite meshes and does the tree have World Position Offset enabled?
Is this similar or different from the issue described [Content removed] In that case proxy shadows might help.
Lastly, would you be able to provide a minimal repro scene containing this asset?
Thanks,
Sam