For background: This is Drupal 8. The menus were fine, but I'm porting the site to Pantheon. My Browser dev tools say that all my files (inc. CSS/JS) are loading properly. There are no JS console errors. My non-Pantheon site doesn't have this behavior, though I've had to make changes to the site to make it work in Pantheon (mainly in Composer) but I can't think of what else could cause this.
Edit: I compared the actual CSS files -
builder.css is different on my non-Pantheon site.