Specular highlights can disappear with clearcoat materials, when "Clear Coat Enable Second Normal" is enabled ("r.ClearCoatNormal").
1. Download the Licensee test project and open (see addl. info URL)
2. Open the level sequence "/Game/SpecularDisappearing/LevelSequence/DemoAnim"
3. Select the Camera to see its preview, and play the sequence.
4. Observe the large fuzzy specular highlight disappears as the difference between the surface normal and the camera vector increases. (Front blue sphere and both back spheres)
5. Observe that, for non-nanite meshes, having anything connected to the Anisotropy output resolves the issue (Front green sphere)
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-203751 in the post.