Quick answer: Unreal Niagara mesh-based particles casting detached shadows? Default shadow bias is tuned for static meshes - lower the renderer's shadow bias or disable shadow casting for VFX.
Smoke particles cast shadows. The shadow sits 30cm below each particle - clearly detached.
Lower shadow bias
Niagara Mesh Renderer > Cast Shadow > Shadow Bias = 0.1. The static-mesh default of 0.5 over-biases dynamic particles.
Disable shadow casting
For most VFX, shadow casting isn't needed. Toggle off; performance improves and the artifact disappears.
Use planar shadows
For ground-shadow stylized effects, a textured plane parented to the particle is cheaper and more controllable.
“Mesh shadow tuning is per-context. Particles aren't static meshes.”
Audit shadow-casting VFX in your project. Most don't need it; turning it off is free performance.