Keeping brightness as set

I often work at a library where I need the screen brighter than at home. When I set it at a higher brightness, however, it won't stay there. An advisor on Apple Chat told me to turn off NightShift, and promised the setting would stay where I put it after that. But it doesn't. Every time the computer goes to sleep—and sometimes for no reason at all—the slider goes back to default. I wind up having to reset it two or three times during a regular working day.

Any ideas on how I can set the brightness to where I want it and have it stay there? I'm running Mojave 10.14.5.