I didn't find the answer to this question, but I did resolve it, so it might be beneficial to share it.
When I use {{issue:key}} and any field in the format {{issue:<<attribute>>}} the email shows a blank space.
When I use {{key}} and any issue field as {{<<attribute>>}} the email shows the field perfectly.
The smart value documentation does not give any clue as to whether to include the context "issue:" or not.
