You should not be modifying message.author here.  getContactData is an asynchronous function, so the line that sets .textContent below this will never see the value of the contact lookup.  This actually also breaks saving the affected message to the database because of a rep-exposure by the back-end.  At least, it looks like structured clone is having trouble serializing the contact name, causing us to not persist to the DB.  This seems like the type of thing that would show up in the course of testing your patch, so please try to pay very close attention when testing.

(I do think it wold be a neat idea to have the contact lookup results already integration into the message.author, but we should be doing that in mailapi.js or in the back-end in that case.)