I am baking a scene with only one grey material, bounced lighting only, one sunlight in mixed mode.
The lightmap UVs are generated on import.

I see stripes in my lightmap when I bake.
The first batches of rays seem ok, but after some time stripes start appearing.
The stripes have changed color and value when I changed the lightmap resolution : they were bright white/yellow with lightmap size 512 and went red and black when I increased to 1024 or more.

It seems that corrupted batches of rays show later when my lightmap size is 4096.

Notes:

- Reproduces on 2018.3.0b4 using GTX 1080