I have noticed that while most people I send emails to or receive emails from (note: they are NOT listed as google contacts) have display names that are either their names or a custom variant, some have display names that exactly match their email address. I always assumed this was because those people had set their display names to match their email addresses, but I asked one of my friends (his email address shows this issue) to check their display name as listed in Gmail settings. They got back to me and told me that their display name was set to their real name. What could be causing this issue? For what it's worth, I checked and their display name had the same problem when I got an email from them to a second email address that I own.
This question has been asked in different forms before, most notably at this link, but the accepted answer to this question is based "on the assumption that [the other person] is not using Gmail" whereas my friend is using Gmail.