Blend shapes in 3D models break (show artifacts like mesh tears or unnatural deformations) mainly because the base mesh’s topology isn’t optimized for smooth transitions, or blend targets have conflicting vertex movements.
Blend shapes depend on identical vertex counts and consistent topology (edge loops, vertex flow) across all targets. If vertices don’t align or move in opposing directions (e.g., a smile pulling vertices one way while a frown pulls another), blending causes inconsistencies—common in facial rigging, where complex expressions need precision.
To fix breaks, ensure all blend shapes match the base mesh’s vertex count/order. Use tools in Blender or Maya to align edge loops for smoother transitions.