I would like to answer the question please try it, it is definitely possible that the typeface Acherus Grotesque doesn't actually support Superscript/Subscript. When other applications encounter fonts that don't support these OpenType features, they oftentimes "fake it" by shrinking the font size and transforming the layer upwards or downwards. We don't do that. Also, currently Figma does not communicate through the UI when/if these features are available or not, so it may appear to "not work".
It's definitely on our list of things we'd love to improve in the future, and as you already know, you got the eyes and ears of Dylan on it, so it's certainly on our radar!