Difference between HISM and ISM in PCG

I installed the road using PCG.

I placed a decal material on the static mesh, and it worked well, but suddenly only the first static mesh is rendered, and the others are not drawn.

In the PCG graph, I changed the component class of the static mesh spawner from hierarchical instance static mesh component to instance static mesh component, and the lines came back to life.
2024-02-22 114304

What potential issues could arise in this case?