Thank you Mr. Kenyon,
It appears to be fine in other apps, such as InDesign and Illustrator—but I just learned it's messed up in other Office apps like PowerPoint as well, so it feels like an Office issue. And after a quick glance, I see at least 60 folders with different versions of Arial, all throughout my computer, in System Folders, resident. If I throw them all away, I'm likely to be in worse shape.