Interpolated vector is not normalized resulting in faceted look when using vertex lights

Steps to reproduce:

1. Download and open the repro project ([Internal link])
2. Open SampleScene
3. Inspect Sphere GameObject

Expected result:

• Indirectly lit faces have a smooth appearance to them

Actual result:

• Polygons have a faceted look to them (see attached screenshot)

Notes:

• Reproduces in all public Unity versions
• Reproduces in Built-in RP only; other render pipelines are unaffected
• The culprit appears to be a non-normalized interpolated vector between vertex normals