How to reproduce:
1. Open the project attached by the user (LightTest.zip)
2. Open the "LightTest" Scene
3. In the Scene View, navigate to the inside of the "Cube" GameObject
4. Notice how the Quads inside are lit up incorrectly and display artifacts

Expected result: The Quads inside are not lit up at all
Actual result: The Quads inside incorrectly and unevenly lit up

Reproducible with: 2017.4.0f1, 2017.4.12f1, 2018.1.9f2, 2018.2.11f1, 2018.3.0b4, 2019.1.0a3

Notes: The problem persists on the "Progressive" Lightmapper settings, but does not reproduce with the "Enlighten" setting. When setting the Lightmapper to "Enlighten", the first bake will still produce the problem, but moving the light a little bit seems to fix it on subsequent bakes.
The rotation of the Light and the Quads doesn't seem to make an impact on the problem, despite what the user reported.
The problem seems to only reproduce with a Cube GameObject, as when the Quads are in other kinds of GameObjects, their lighting behaves normally.