I am still gathering evidence but from what I can tell, it makes all of your linking to yourself nofollow unless Medium whitelisted you as a dofollow domain.

I've been trying to get in touch with support on that matter but it hasn't been fruitful. Sucks because I really like Medium but my SEO tanked come 2020.

The Wayback Machine shows that all Medium links were dofollow until the end of 2019 then the made the change. This would explain my SEO drop.

All things considered though, I think that custom domains as they are now are quite bad for SEO...