Quick answer: Unreal Niagara mesh emitter billboard mode facing the wrong direction? Facing mode interprets mesh forward; mesh's local axis may not be Y - audit the mesh's forward.
Sprites face camera; mesh particles face the mesh's intrinsic forward (sometimes wrong direction).
Reorient the mesh
Edit mesh in DCC; align Y axis with intended forward.
Or use Facing Vector
Custom facing vector overrides mesh's intrinsic forward.
Test billboard
Move camera around; mesh should face. Visible misalignment = the bug.
“Mesh orientation has conventions. Billboard reads the convention.”
Document the mesh orientation convention. Asset import follows; visual bugs avoided.