Tangent normals are recalculated by default on skeletal meshes with blend shapes

In 4.13 if you imported a skeletal mesh with a blendshape and run the blend shape. The normals are not recalculated.

To Recalculate the normals during runtime you need to edit the Console Variables.ini and add

r.SkinCache.CompileShaders=1
r.SkinCache.Mode=1
r.SkinCache.RecomputeTangents=1

And then on the skeletal mesh you have to enable Recompute Tangent.

In 4.14 release.
Without enabling anything, Skin cache or the recompute tangent (Skin cache needs to be on for this to work) tangents are recomputed on blendshapes anyway.

1. Download the release build and open the editor
2. Create a new blank project
3. Import the attached file with Import Morph Targets enabled
4. Add the animation Smooth_sine_Anim to the gameworld
5. Change the view mode to World Normal
6. Hit simulate
   Result
   As the animation plays the normals are recalculated without SkinCache turned on Or Recompute tangent enabled on the skeletal mesh