Repro
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1.Download the attached project: https://oc.unity3d.com/index.php/s/89gVTjeNAhU2i4D
2. Open the Scene MasterScene_00 (Baked GI).unity.
3. Clear baked data and GI Cache.
4. Select the GameObject AdditiveSceneLoader from the Hierarchy.
5. In the Inspector, click the Additively Load All Scenes button. This will load the complete Scene "set".
6. From the Lighting Settings window, click Generate to initiate a lighting precompute. Wait for this to finish.
7. In the Hierarchy select 'Ground' gameobject.
8. In the Lighting window double-click on the lightmap highlighted in yellow.
9. In the opened Lightmap Preview window expand the dropdown list and select 'Texel Validity' visualization mode.
10. Observe that Lightmap UV grid is aligned with Lightmap UV itself as expected (see attached 'before' screenshot).
11. Close the Lightmap Preview window.
12. Select the GameObject AdditiveSceneLoader from the Hierarchy.
13. In the Inspector click 'Unload All Scenes' button.
14. In the Hierarchy select 'Ground' gameobject.
15. In the Lighting window double-click on the lightmap highlighted in yellow.
16. In the opened Lightmap Preview window expand the dropdown list and select 'Texel Validity' visualization mode.

Observed
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The UV overlay is completely mismatched to the underlying UV overlap preview (see attached 'after' screenshot). If the Scene View mode is also set to UV Overlap, this will also become corrupted This makes it very difficult to use the preview for any kind of diagnosis. In the attached screenshot the UV overlap preview is actually from an entirely different object in the Scene.

Expected
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UV overlay and the underlying image should match. Maps should be from the selected object only.

Notes:
- Reproducible in 2018.3.0a6, 2018.2.1f1, 2018.1.9f1;
- Reproducible on Windows 10 and OSX 10.13.6;
- The issue doesn't reproduce when auto-mode is set in the Lighting window.