Quick answer: Unity importing a mesh with blendshapes produces scrambled deltas? The DCC tool reordered vertices on export — blendshape deltas no longer map to the same vertices.
An updated head model imports with blendshapes that twist randomly. Blender re-ordered vertices when applying a modifier.
Lock Vertex Order
In Blender: Object Properties → Object Index, or use the FBX exporter’s “Keep vertex order” option. In Maya: Mesh → Cleanup with appropriate settings.
Normalize Blendshapes
Unity Import Settings → Animation → Normalize Blend Shapes. Helps in some cases but can’t fix a true reorder — needed only when the source has expected normalization issues.
Re-Author Blendshapes
If vertex order is lost, regenerate blendshapes from the base mesh. Tedious; sometimes the only fix when the DCC re-orders silently.
Apply Modifiers Carefully
Applying mesh modifiers (subdivision, decimation) re-orders vertices. Apply before authoring blendshapes; never after.
Verifying
Blendshapes apply correctly — expected facial expressions, no twisted geometry.
“Vertex order is fragile. Lock it in the DCC; never apply mesh modifiers after blendshapes exist.”
Document the export pipeline in the art repo — the ‘don’t re-order’ rule is easy to forget months later.