https://github.com/IBM/plex/blob/a1a84378d19f8a86fde19e228574550281e48b3b/IBM-Plex-Sans-Variable/fonts/split/woff2/IBM%20Plex%20Sans%20Var-Roman.css#L5 restricts the font to the weight `400`. It should be `font-weight: 100 700`. It should also have `font-stretch: 85% 100%` for the other available axis.