Morph target normals not correctly applied when imported as LOD 0

This is a regression since 5.5 with skeletal meshes where morph target normal/tangent deformation only works when a mesh is imported as a LOD of an existing skeletal mesh, but not when imported as LOD0.

• Importing a mesh with morphs as LOD0: morph positions work, but normal/tangent deformation does not affect shading.
• Importing the exact same mesh as LOD1/LOD2 of the same asset - morph normals/tangents work correctly.
• Import the same mesh as a separate skeletal mesh - morph target normals also do not work.

In UE 5.5

• Import the attached FBX as a Skeletal Mesh with default settings.
• Set the morph target value to 1.
• Result: the cube shape is smoothed and the normals follow the morph as expected (shading changes correctly).

In UE 5.6/5.7/5.8

• Import the same FBX as a Skeletal Mesh with the same default settings (no changes from 5.5).
• Set the morph target value to 1.
• Result: the cube shape is smoothed, but the normals stay unchanged (shading does not react to the morph).
• Open the imported mesh, go to LOD Settings and set the number of LODs to 2.
• Reimport the same FBX as LOD1.
• Result: on LOD1, the morph affects the normals correctly (shading changes), while on LOD0 it still does not.