If I set the directory settings in 'General' so that when using the directory, it will search through both the first and last names of people the following behaviour is seen:
If I enter no digits in the phone, instead of looping to a second repeat of the instructions, the call is connected to the voicemail of a user with a last name beginning with Z and so is the last user when sorted by last name.
If I add other people with last names starting with Z, the original user is still used, but the AGI now asks if this is the correct extension, and if one presses the * key, it says there are no more compatible entries and goes back to the beginning.
For now, I have switched back to using only the last name.
FreePBX 184.108.40.206 is in use.