Steps to repro:

1. Open the attached project and 'cube' scene;
2. Observe that cubes using custom emissive meta pass shaders contribute emissive lighting to the plane;
3. In the Lighting window change Lightmapper to 'Progressive';
4. Wait until the scene re-bakes.

Expected result:
Cubes using custom emissive meta pass shaders contribute emissive lighting to the plane.

Actual result:
Cubes using custom emissive meta pass shaders don't contribute emissive lighting to the plane after changing lightmapper to PLM. See attached video.

Notes:
- Reproducible in 2018.3.0a3, 2018.2.0b11, 2018.1.6f1;
- Reproducible on Windows 10;
- Reproducible only for PLM; not reproducible for Enlighten.

It is part bug, part authoring error, as the material is set up to do realtime GI emission:

Adding this in the PropertyBlockTest.cs script:
print("Material.globalIlluminationFlags: " + rendererTest.sharedMaterial.globalIlluminationFlags);
results in `Material.globalIlluminationFlags: RealtimeEmissive`

Adding this line, will make it bake correctly with the progressive lightmapper:
rendererTest.sharedMaterial.globalIlluminationFlags |= MaterialGlobalIlluminationFlags.BakedEmissive;