Here's the scenario. Each night before bed I plug my laptop (2016 MacBook Pro running macOS Mojave) into the power adapter. I put on an album in iTunes and and dim the display till it's completely black. After about ten minutes each night, the display suddenly gets bright, which I can see because the lights are off in my room. The screen stays bright for about 15 or 30 seconds, then goes back to black. All the while the music keeps playing.

What gives? Any way I can make it so the screen doesn't brighten for those 15-30 seconds?

To stop your MacBook from automatically adjusting the screen brightness level:
Click the Apple icon in the upper-left corner of your screen.
Select System Preferences.
Click Displays.
Uncheck "Automatically adjust brightness."

Greetings,
Peter