Less brightness after unlocking the screen

I have an Asus ZenBook 13 OLED (UM325SA) with AMD Ryzen 7 5800U and Windows 10. After locking the screen and turning off the display, the screen appears visibly dimmer when unlocked, even though it shows as "maximum brightness". If I decrease the brightness by one step, the screen immediately becomes brighter. This is a brand-new system set up just yesterday. Whether it’s plugged in or not makes no difference.