iPhone dims halfway, even with Auto-Brightness off

My iPhone brightness periodically dims by itself. It seems like it's always to exactly half-brightness. I'm not sure how to stop it from happening. It think it usually happens while I'm not using the device; that is: I'll notice it's been dimmed when I go to use it after a short break.

Auto-Brightness in the Accessibility features' Display & Text size options is off. True Tone in Display & Brightness options is off. Reduce White Point is off. Night Shift is off. Low Power Mode is off. I've even gone through and cycled them all on/off to make sure I'm not missing something.

I've been digging around the internet for over a week trying to find a solution. The only two possible answers I've found are:

1. Some folks have mentioned something to do with the phone overheating... this doesn't seem likely in my case, the phone isn't undergoing extreme use, no weird Beta or under Development apps, not in direct sun or a hot room or anything like that.
2. Some folks have mentioned the "Attention Aware" features may impact brightness. I guess that's possible, but I'm not sure why it would since Auto-Brightness is off anyway. But maybe it's dimming when I put the phone down, and then doesn't brighten back up when I start using it again?
3. Only other unique piece to my situation is my phone connects to a VPN for work, but I don't see how or why that would impact brightness settings.

I'm running iOS 14.3 on an iPhone 12 mini. If anyone has any insights, I'd love to hear them! It's been frustrating needing to re-adjust my brightness every time I go to use the device! Thanks!