LookAt は DefaultExpressionValidator.Validate よりあとで処理されて IsBinary の閾値が 0 であった

see DefaultExpressionValidator.Validate
var weight = expression.IsBinary ? Mathf.Round(inputWeights[key]) : inputWeights[key];

Assets/VRM10/Runtime/Components/Expression/ExpressionMerger.cs

@@ -63,7 +63,7 @@ namespace UniVRM10
 
             if (clip.IsBinary)
             {
-                value = value > 0 ? 1 : 0;
+                value = value > 0.5f ? 1 : 0;
             }
 
             m_morphTargetBindingMerger.AccumulateValue(key, value);