Notifications appear in incorrect corner of screen

When I get notifications on macOS High Sierra 10.13.6, they appear in the upper left corner of the screen, rather than the upper right. Additionally, they're too high — just below the menu bar (or slightly overlapping). I thought the issue might be Magnet but even when I quit Magnet, the notifications appear in the upper left corner. What's going on? Do I have an app installed that's breaking notifications, or is this a High Sierra bug?