Lightmap UV mismatch in 2017.2 and 2017.4

Background: We have a tool that bakes and stores the baked lights of a prefab onto itself (has been working fine in 2017.2.0f3). T$$anonymous$$s way, when we export the prefab, the lightmaps will be exported together with it. We have no issues importing t$$anonymous$$s prefab (via .unitypackage) into other 2017.2.0f3 projects; the prefab auto-applies the lightmaps successfully onto itself in any scene it is instantiated in (both editor and run-time).

Issue: We have just upgraded our project to 2017.4.17f1, and the lightmap UVs of the same prefabs seem to have been modified by Unity, resulting in our assets looking broken. We're trying to avoid re-baking a tonne of prefabs all-over in 2017.4, so any help would be much appreciated.



We've tried toggling on/off the Generate Lightmap UVs setting in 2017.4.17f1 at model's import settings. Toggling off seems to product a set of lightmap uvs that is closer to the original. But either ways, the UVs are still wrong.

Based on t$$anonymous$$s screenshot, the issue should be with the altered lightmap uvs processed by Unity rather our prefab lightmap tool.