Steps to repro:

1. Open attached project and 'ProgressiveLightmappingExample' scene;
2. Go to Lighting Settings window (Window>Lighting>Settings);
3. Click 'Generate Lighting' window;
4. Wait until you see 'Baking [ETA]' stage in the lightmap baking progress bar;
5. Select any gameobject having a material in the scene;
6. Expand gameobject's material in the Inspector.

Expected result:
Lightmap baking continues from the same stage.

Actual result:
Lightmap baking is started from the beginning. See attached video.

Notes:
- Reproducible in 2018.1.0a6, 2017.3.0f1, 2017.2.0p3;
- Reproducible on Windows and macOS;
- Reproducible only for PVR; not reproducible for Enlighten;
- Reproducible only once when expanding certain material, after that, it won't repro with the same material;
- The bug to be specific to this project; could not repro from scratch.