One-sided mesh renderers (e.g. a default Unity plane or a mesh imported from an external 3d software) can't block out light emitted by a mesh with emissive material or a Light when the lighting is baked. The light passes through to the other side of mesh. The result is the same when "Cast Shadows" parameter of Mesh Renderer is set to "Two-Sided".

Steps for reproduction:

1) Open "TestScene1".
2) Ensure that in "Lighting" window, the "Ambient GI" parameter is set from "Realtime" to "Baked".
3) Build the lighting.
4) Observe the bug.

Notes:
- Alternatively, the cube with emissive material can be disabled and "Point light" can be enabled in the scene. The type of light can be changed to anything. Same bug can still be observed.
- In GIF attached, Shaded Mode and Baked Mode in the editor are displayed.
- Reproduced with the version 5.3.5p6, 5.4.0b22, 5.4.0b25, 5.5.0a3