If I call e_cal_client_get_object_sync(client, "20080407T193125Z-19554-727-1-50@gollum", NULL, &comp, NULL, &gerror), I get the following result:
***
BEGIN:VCALENDAR
PRODID:-//Ximian//NONSGML Evolution Calendar//EN
VERSION:2.0
BEGIN:VEVENT
UID:20080407T193125Z-19554-727-1-50@gollum
DTSTAMP:20080407T193125Z
DTSTART:20080413T090000Z
DTEND:20080413T093000Z
TRANSP:OPAQUE
SEQUENCE:7
SUMMARY:Recurring: Modified
CLASS:PUBLIC
CREATED:20080407T193241
LAST-MODIFIED:20080407T193647
RECURRENCE-ID:20080413T090000Z
DESCRIPTION:second instance modified
END:VEVENT
END:VCALENDAR
***

Instead of:

***
BEGIN:VCALENDAR
PRODID:-//Ximian//NONSGML Evolution Calendar//EN
VERSION:2.0
BEGIN:VEVENT
UID:20080407T193125Z-19554-727-1-50@gollum
DTSTAMP:20080407T193125Z
DTSTART:20080406T090000Z
DTEND:20080406T093000Z
TRANSP:OPAQUE
SEQUENCE:2
SUMMARY:Recurring
DESCRIPTION:recurs each Sonday\, 10 times
CLASS:PUBLIC
RRULE:FREQ=WEEKLY;INTERVAL=1;BYDAY=SU;UNTIL=20080608T090000Z
CREATED:20080407T193241
LAST-MODIFIED:20080407T193241
END:VEVENT
END:VCALENDAR
***

It returns an instance instead of returning the parent. I'm not able to reproduce the issue by using the deprecated ecal API (e_cal_get_object).

I compared the code for e_cal_client_get_object_sync() and e_cal_get_object() and I found the following difference:

e_cal_client_get_object_sync() is calling complete_get_object_master() which does:

***
} else if (res && icalcomponent_isa (tmp_comp) == ICAL_VCALENDAR_COMPONENT) {
                                // XXX: Why this loop?
                                for (master_comp = icalcomponent_get_first_component (tmp_comp, kind);
                                     master_comp;
                                     master_comp = icalcomponent_get_next_component (tmp_comp, kind)) {
                                        if (!icalcomponent_get_uid (master_comp))
                                                continue;

                                        if (icaltime_is_valid_time (icalcomponent_get_recurrenceid (master_comp)) &&
                                            !icaltime_is_null_time (icalcomponent_get_recurrenceid (master_comp)))
                                                break;
                                }

                                if (!master_comp)
                                        master_comp = icalcomponent_get_first_component (tmp_comp, kind);

                                *icalcomp = master_comp ? icalcomponent_new_clone (master_comp) : NULL;
                        }
***

The equivalent in e_cal_get_object() does not have the for loop and calls icalcomponent_get_first_component() directly. If I comment the for loop, the method returns the expected result.

In the for loop, it appears we are making sure that master_comp has a recurrence ID but I don't think this is correct when the client asks for a parent (with RID == NULL).