The Guild Applicants Disabled Bug causes the Guild Applicants and Applicant History drop-down options in the Blizzard Guild Roster tab to be greyed-out,
which prevents the display of the Guild Applicants for a guild officer.
The orange diamond is displayed indicating that new guild applicants exist, but the bug prevents the applicants from being displayed.
Some guilds, and all their guild officers, have this bug, which is very frustrating to not be able to view and invite new guild applicants.
This bug has not been fixed yet by Blizzard after several months, so it seems that there will be no bug fix soon.
As a temporary workaround, this GuildApplicantsFix addon will automatically fire a CLUB_FINDER_APPLICATIONS_UPDATED event whenever a GUILD_ROSTER_UPDATE event occurs.
This will enable the drop-down list options for Applicants and Applicant History, so that they are not greyed-out. This allows the Guild Applicants list to be viewed and actioned by a guild officer.
The addon will auto-refresh the Guild Applicants when a new applicant is added to the list or when an applicant is invited by a guild officer and removed from the list.

Note: Click on the Blizzard Guild Roster tab to cause the guild roster and guild applicants to be loaded. The Applicants drop-down options will then be enabled with this addon installed.

As an alternative to this addon you can also make a one-line macro containing:

/script CommunitiesFrame:OnEvent("CLUB_FINDER_APPLICATIONS_UPDATED");

Then bind the macro to a key-combo or drag the macro to an action bar slot, so you can key-press or action-button-click whenever you want to refresh the Guild Applicants display.

However this GuildApplicantsFix addon will do the applicants refresh automatically for you, so that you do not need a macro and do not need to press a macro button to refresh the Guild Applicants display.

The GuildApplicantsFix addon is open-source maintained on GitHub here:

GitHub GuildApplicantsFix Source

Guild Applicants Fix