I thought that defaults to being off, or at least it was on mine aside from Quick Start Transfer settings from mini 5 and upgraded to 15.1 b3. But yeah that can make it dim.
I setup the mini 6 as new and I’ve just been adding things as I go along. I don’t use it for all the same things as my bigger iPads so I figured I’d just setup as new to minimize the crud.
I normally have the Accessibility shortcut for Reduce White Point in Control Center on my devices so I can quickly enable/disable depending on the environment. Hadn’t added it to the mini 6 yet so entirely possible I manually enabled it during night time reading and forgot to disable after.