Exchange Online EWS Only Returns Availability when MinimumSuggestionQuality is set to Poor - Why?

Question

We are using impersonation to access mailboxes hosted in Office 365 using EWS but the app will only return available timeslots when the availabilityOptions.MinimumSuggestionQuality = SuggestionQuality.Poor. I've tried adjusting the availabilityOptions.GoodSuggestionThreshold
to no avail. Beyond assigning the "ApplicationImpersonation" role to the account being used to access mailboxes, are there any other permissions, licenses, or attributes that need to be set in order to use EWS to query for free/busy time? 
The code snippet is below.
        //Returns a list of TimeSuggestions for the specified user(email) and date.
        string MyUser = "[email protected]";
        //LB Note: Pulled impersonation information from https://msdn.microsoft.com/en-us/library/office/dn722379(v=exchg.150).aspx#sectionSection1       
        service.Credentials = new NetworkCredential("[email protected]", "acns_calendarapptestpw");
        service.Url = new Uri("https://outlook.office365.com/EWS/Exchange.asmx");
        //Changing MyUser to an invalid user ([email protected]) or a valid user (o365test100) that the app doesn't
        //have impersonation rights to will throw an error stating as much
        service.ImpersonatedUserId = new ImpersonatedUserId(ConnectingIdType.PrincipalName, MyUser);
        List<TimeSuggestion> suggestions = new List<TimeSuggestion>();
        //LB Note: I pulled this example from https://msdn.microsoft.com/en-us/library/office/dn643673(v=exchg.150).aspx
        List<AttendeeInfo> attendees = new List<AttendeeInfo>();
        attendees.Add(new AttendeeInfo()
            SmtpAddress = MyUser,
            AttendeeType = MeetingAttendeeType.Organizer
        // Specify options to request free/busy information and suggested meeting times.
        AvailabilityOptions availabilityOptions = new AvailabilityOptions();
        availabilityOptions.GoodSuggestionThreshold = 10;
        availabilityOptions.MaximumNonWorkHoursSuggestionsPerDay = 1;
        availabilityOptions.MaximumSuggestionsPerDay = 30;
        availabilityOptions.MeetingDuration = 60;
        // LB Note: Setting this to "Poor" was the only way I could get results to show. Not sure what's going on here.
        availabilityOptions.
        //Returns a list of TimeSuggestions for the specified user(email) and date.
        string MyUser = "[email protected]";
        //LB Note: Pulled impersonation information from https://msdn.microsoft.com/en-us/library/office/dn722379(v=exchg.150).aspx#sectionSection1       
        service.Credentials = new NetworkCredential("[email protected]", "acns_calendarapptestpw");
        service.Url = new Uri("https://outlook.office365.com/EWS/Exchange.asmx");
        //Changing MyUser to an invalid user ([email protected]) or a valid user (o365test100) that the app doesn't
        //have impersonation rights to will throw an error stating as much
        service.ImpersonatedUserId = new ImpersonatedUserId(ConnectingIdType.PrincipalName, MyUser);
        List<TimeSuggestion> suggestions = new List<TimeSuggestion>();
        //LB Note: I pulled this example from https://msdn.microsoft.com/en-us/library/office/dn643673(v=exchg.150).aspx
        List<AttendeeInfo> attendees = new List<AttendeeInfo>();
        attendees.Add(new AttendeeInfo()
            SmtpAddress = MyUser,
            AttendeeType = MeetingAttendeeType.Organizer
        // Specify options to request free/busy information and suggested meeting times.
        AvailabilityOptions availabilityOptions = new AvailabilityOptions();
        availabilityOptions.GoodSuggestionThreshold = 10;
        availabilityOptions.MaximumNonWorkHoursSuggestionsPerDay = 1;
        availabilityOptions.MaximumSuggestionsPerDay = 30;
        availabilityOptions.MeetingDuration = 60;
        // LB Note: Setting this to "Poor" was the only way I could get results to show. Not sure what's going on here.
        availabilityOptions.MinimumSuggestionQuality = SuggestionQuality.Poor;
        availabilityOptions.DetailedSuggestionsWindow = new TimeWindow(Convert.ToDateTime("4/1/2015"), Convert.ToDateTime("4/2/2015"));
        availabilityOptions.RequestedFreeBusyView = FreeBusyViewType.FreeBusy;
        // Return free/busy information and a set of suggested meeting times.
        // This method results in a GetUserAvailabilityRequest call to EWS.
        GetUserAvailabilityResults results = service.GetUserAvailability(attendees,
availabilityOptions.DetailedSuggestionsWindow,
AvailabilityData.FreeBusyAndSuggestions,
availabilityOptions);
= SuggestionQuality.Poor;
        availabilityOptions.DetailedSuggestionsWindow = new TimeWindow(Convert.ToDateTime("4/1/2015"), Convert.ToDateTime("4/2/2015"));
        availabilityOptions.RequestedFreeBusyView = FreeBusyViewType.FreeBusy;
        // Return free/busy information and a set of suggested meeting times.
        // This method results in a GetUserAvailabilityRequest call to EWS.
        GetUserAvailabilityResults results = service.GetUserAvailability(attendees,
availabilityOptions.DetailedSuggestionsWindow,
AvailabilityData.FreeBusyAndSuggestions,
availabilityOptions);

Answer 1

For EWS to work with App Token, you need to do two more things. One you already did: Set the impersonation header. Two is to set the X-AnchorMailbox http request header to the smtp address of the mailbox you want to access. This routes the request instantly
to the right backend.
Hope this helps.
Thank you! Matthias