4K Lightmaps look corrupted when generating lighting with Progressive Lightmapper in auto mode

Steps for reproduction:

1) Download the project below and open the scene `s1`
or
Create lightmap static cube and plane with a baked light
2) Ensure that lightmap resolution is higher than 70 and lightmap size is set to 4096
3) Generate lighting
4) Observe the bug

Notes:
- Not a regression, it was always like that since the introduction of progressive lightmapper