I have a 2017 Accord Sport. It seems that, when it was new, the display illumination would brighten and darken with ambient light (when light switch is on automatic). Not doing that now.

Was I imagining this, or is the car's display illumination supposed to change with ambient light?

 

You may want to delve into the menu to look for lighting options. Also, there is a brightness button that toggles between lighting modes.