All editor fonts showing up as Helvetica

Hi, I'm running RStudio Desktop on Ubuntu 17.10 and I'm running into a frustrating issue. The editor font is always rendered as Helvetica, regardless of which fixed-width font I choose in the options menu. The issue happens on both Xorg and Wayland and it's really driving me nuts (as the cursor moves around independently of the text, as if the fixed-width font were actually there.)

The closest issue I've seen to my problem is this one, but it seems very much like some component of RStudio simply cannot find the font I want, even when it shows up in the editor font menu. Even manually editing the font through inspect-element doesn't fix the issue.

Does anyone know what's going on here?