iPad 9.7 + iOS 12 = No auto-dimming

[jon3543]

I just bought a new iPad 9.7 2018 model and installed iOS 12 when I set it up. I don't know if Auto Brightness worked in iOS 11, but only half of it works for me in 12. That is, while the screen will brighten in bright ambient light conditions, it simply will not dim, even in a pitch black room. Besides simply adjusting the brightness in dark conditions and finding that it doesn't stick, I've also tried turning Auto Brightness off and adjusting brightness, then turning it back on, and that doesn't help. Any ideas?

ETA: The iPad auto-locked after I wrote the above, and when I unlocked it, the screen was dimmer. Moved it back to bright lighting, and it brightened up. Moved it back to low lighting, and it failed to dim. This is reliably reproducible. It only gets dimmer when unlocked, and then it never auto-dims again as long as it's awake.

 

[jon3543]

With my Auto-Lock set at 5 minutes, the screen dims completely a little after 4 minutes prior to auto-locking. When I tap the screen to prevent it from locking, the brightness comes back up to the level it should have auto-dimmed to during that 4 minute interval. That is, when I started this little test, I held the iPad under bright light to jack up the brightness. Then I removed it from the light and watched the brightness stay at that high level. Only after it dimmed completely prior to auto-locking did the auto-dim kick in when I tapped the screen.

My iPhone OTOH starts dimming immediately when removed from the light. The iPad behavior is wrong.

Anyone?