This was the real issue. I was not familiar enough with typography to catch my mistake, so I assumed it was a problem with the program. If it gave a notice for why it did not render the font, I would not report it as an error.
The tool itself doesn’t render anything - it generates a TTF file, converts it to base64, and hands it over to the browser to use. Browsers, unfortunately, do not commonly display any warnings about why the font cannot be loaded and may rely on something else for font loading/rendering as such.
Aside of verifying that the inputs are hypothetically correct, not really - even just figuring out whether the browser is rendering the resulting font wrong (let alone why it’s doing that) is problematic.
