Just double-checked. Thunderbird actually doesn't behave completely correctly - in the message list, it shows
Rafael =?iso... but in the message display it shows
Rafael Fernández... That could theoretically be a weird protection against phishing, though, and not an accidental bug.
Also, doing some more digging, these messages
do show up properly in Aquamail when they're received through a Groupwise IMAP server. The Groupwise-received messages which work properly in Aquamail also work properly in both message list and message display in Thunderbird. It might actually be something extra on my end confusing things, or it might be that Rafael is composing the Groupwise messages in mutt on his workstation, while GitHub is doing something differently when it generates messages.
I'm having a harder time finding an example where this happens and where it was generated by something other than GitHub.