Is there any chance that anyone has tried contacting them via their support e-mail address?
We take a break on the weekends (JST), but during weekdays we aim to reply within 1-2 business days. If you are experiencing a problem that our support section doesn’t solve please reach out to us!
Details of the general information they require can be found on the support page.
While there is obviously the issue on the GitHub repo, that repo does not look to be actively maintained at the moment. Probably because a font is generally pretty stable and they don’t use GitHub much, or the developer for that may have moved on from the organisation, etc. However, the general app support for iA Writer would seem a good next step.
I note also that the repo just seems to be the final font files. I don’t see any font generation resources in the repo, so I think it would have to be the creators who fix it unless someone fancies rebuilding the font from scratch. Well, almost scratch, the IBM Plex font resources it is based on are available.
Failing that, the iA Write folks do look to be active on their Subreddit (https://www.reddit.com/r/iawriter), but I don’t see any current mention of Quattro font issues on their, and that might be another avenue to pursue if the e-mail support does not pan out.
Failing that, I’m sure there are alternatives out there that people could find equally as good and will not suffer from this issue.