Light leak on mesh when in shadowed area

When Cast Shadow and Self Shadow Only in the Lighting Section of the Details of a mesh, the light will randomly hit the mesh even though it is in a shaded area. Light appears to flicker and will come and go as the HandMesh is moved throughout the area. Difficult to capture in screenshot, sample project attached.

Reproduced in 4.17.2 CL 3658906, 4.18.0 CL 3709383, and 4.19 Main

1. Create new VR Project
2. Add LightmassImportanceVolume
3. Create shadowed area using geometry
4. In BP_MotionController, navigate to the viewport and select the HandMesh Component
5. In the Details Panel, enable Cast Shadow in the Lighting section and enable Self Shadow Only
6. Play level in VR Preview, navigate to shadowed area

Result: HandMesh remains shadowed but with some patches of light
Expected: HandMesh remains shadowed with no visual discrepancies