Steps to reproduce:

1. Open the updated CornellBox project
2. Open the Test Runner Window (Window > General > Test Runner)
3. Click Run All
4. Observe the Scene View

Reproduces in: 2019.1.0a14, 2019.1.0b1, 2019.3.0a3, 2019.3.0a10, 2019.3.0b8, 2019.3.0f5, 2019.3.7f1, 2020.1.0b1, 2020.2.0a4, 2020.2.0a5, 2020.2.0a6
Could not reproduce with: 2019.1.0a10, 2019.1.0a5, 2019.1.0a1, 2018.2.21f1

Could not repro on 2020.2.0a12 because of a bake looping bug currently under investigation.

Note: This only reproduces with the GPU Lightmapper, and with Progressive Updates ON.

If you're trying to repro the bug and iterate fast, it is recommended to use the lowest possible sample counts, GPU PLM and no filtering.