Iphone dims dramatically, with auto adjust off

My phone is not hot in the least. I just started using it. I noticed this new issue which just started a few days ago. Auto-adjust display brightness is off, yet my screen dims quite dramatically and brightens again and again within a minute. Ambiance is still on. Happens when playing the simplest of braindead games but only at a time when nightshift is also on. I am wondering if there is a newly introduced software issue or a virus?