Xperia Z3 :: After Unlocking Phone From Standby Brightness Is Noticeably Lower

After unlocking the phone from stand-by the brightness is noticeable lower than the one I set, without adaptive brightness on, and after a short period of time, usually under 30 sec, the brightness goes up to the level I set it, and stays like that as long as the display is on. This problem repeats randomly, but very often, maybe 80% of the cases. The difference in display brightness when this happens is probably 10% - 15% and is very annoying. I already did a factory reset and a fresh install via PC Companion with no results. The phone is 2 months old, no major software update until now so I can't tell if it's a software or hardware problem.