When I moved a ship to join another ship and form a fleet (not necessarily the trigger), the 'Command' control disabled, preventing me from issuing a 'Guard', 'Sentry' or other command, forcing me to move or pass to clear the 'Idle Ship' end-turn-blocker.
I saved, re-loaded and the very same fleet's "Command" controls re-enabled, allowing me to put the fleet on Guard.