[p3lim]

Here's what I use, seems to work fine for me atleast:

local lastMissionID
local function QueryMissions()
    local missions = C_Garrison.GetCompleteMissions()
    if(#missions > 0) then
        lastMissionID = missions[1].missionID
        C_Garrison.MarkMissionComplete(lastMissionID)
    else
        GarrisonMissionFrame.MissionTab.MissionList.CompleteDialog:Hide()
    end
end
 
Handler('GARRISON_MISSION_NPC_OPENED', QueryMissions)
Handler('GARRISON_MISSION_COMPLETE_RESPONSE', function(missionID, canComplete, success)
    if(missionID == lastMissionID and canComplete) then
        if(success and C_Garrison.CanOpenMissionChest(missionID)) then
            C_Garrison.MissionBonusRoll(missionID)
            return
        end
 
        C_Timer.After(1/2, QueryMissions)
    end
end)