[throwaway25239]

The old domain was a *.github.io domain (which I migrated to a custom domain), so I don't think it's possible to use my own server for it.

Google did pick up the relationship correctly, at least in the beginning - the new domain got the same ranking as the old one, and the "backlinks" section of the search console shows links to the old domain under the property information for the new domain. So it seems like this type of redirect does have an effect, unless it somehow expires after a few months and a 301 redirect doesn't?