Well, answering my own question here, after renaming the field in question, which I had called “email”, into “emailAddress”, GraphQL found the field and my frontend user interface appeared again. Go figure. Apparently between version 4.10 and 4.13, “email” became a bad name to use for your custom content type fields.
It would be lovely if illegal field names were warned about, either during the build phase, or when saving / modifying the same in the Content Type Builder. Would have saved me 2 days of my life lol…