No need to report anymore. From SeedyL’s dev blog:
Over the last several days, we’ve noticed many reports about an issue affecting PC players where the sim will always open in windowed mode even if full screen mode was set. Players using both the regular release build and the SU_15 beta build are affected by this issue. The team has identified the source of this bug, and a fix will be coming with SU_15. In the meantime, the workaround to restore full screen mode is to set this option in your General Settings each time you launch the sim or simply press Alt+Enter on your keyboard.