Users often notice that letters are missing from the navigation in the Search bar when adding attendees. For example, A, D, E, F appears instead of the complete alphabet.
The list is dynamic and based off of the first 50 results returned. For example, if there are no last names that begin with C in the results, that letter does not display to the user. If the user hits 50 results before it gets to the end of the alphabet, then the user does not see any of the letters that are not in the first 50 results.
This is working by design. The user can use the Search bar to locate accounts that are not returned in the first 50 results.