Thanks for your report! This is unfortunately a known issue with Preview 15 and will be fixed with the next Preview release.
Something is messed up with lighting in the Enscape window live view in preview 15. See comparison of normal view in preview 14, and weird sections with incorrect lighting calculations in v15. As I turn the view I can see alternating patches of correct and incorrect lighting.
I'm looking forward to the next preview release, as the light bleed and errant shadows are making it tough to do interior renders. Any expectations on the next release with possible lighting corrections?