This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
On Her Majesty's Secret Service
The death of Tracy caused Bond to resign from MI6 out of grief.
If you subscribe to the fan theory that James Bond is a code name and several agents have used it (explaining Bond's long career and changes in appearance caused by multiple actors), then it's possible that the grief over Tracy's death caused Lazenby-Bond to resign from MI6, or he was declared psychologically unfit to continue. This resulted in Connery-Bond returning to duty for Diamonds Are Forever before being replaced by Moore-Bond.