Steps to reproduce:
1. Open user-submitted project
2. Drag _TMP/Inner 2 prefab into the Scene
3. Go to Window->Lighting->Settings (or Window->Rendering->Lighting Settings)
4. Change Lightmapper to Progressive
5. Uncheck auto-generate and click Generate to generate the lightmaps

Expected result: no errors and lightmaps do not take this long to generate
Actual result: lightmaps take an abnormally long time to generate, sometimes throwing errors. Artefacts may also appear in generated textures (see screenshots for comparison)

Reproduced with: 2017.1.1f1, 2017.4.7f1, 2018.1.7f1, 2018.2.0f2, 2018.3.0a5

Notes:
- Versions up to 2018.1 throw an error "[PathTracer] AddInstanceProperties job with hash: ... failed with exit code 5."
- The issue does not seem to reproduce on Mac versions of Unity