Internal Resolution Locked to 1080p?

I’m designing a user interface for a 720p resolution, but when I set my assets to conform to 720p, and test the game in both a new PIE window and standalone, the game window appears at 720p, but its internal resolution is still 1080p, which causes the assets to not fill the screen like I want them to. I’ve set every relevant resolution setting in the editor that I could find to 1280x720, but the window still stays locked at 1080p internally. How do I fix it so both the window resolution and internal resolution are in 720p?