It looks like this was the result of automatic codeshare matching performed by the API. Schedule data indicated that once a week Lufthansa Cityline (CLH) would operate the Lufthansa mainline (DLH) flight staring on June 20th. After that date the codeshare to CLH was automatically returned by the API. This revealed old, but already existing, flights that match the codeshare lookup.
We assume, that flight DLH1035 actually took place at 2020-06-14, since it was bookable up to that day. Our question is, it was not reported in the querry on 2020-06-15 but it was reported with the querry at 2020-06-22
Actually this flight is in my opinion not a codeshare, it is never published to the custumer as a CLH flight. I assume that CLH is aircraft owner, since CLH (Lufthansa Cityline) is a 100% subsidiary of DLH (Lufthansa Airlines)
This was just one example. We experienced an increase in number of flights by 10%.
We did querry twice first on 15th of June and than sencond as 22nd of June. The first querry resulted in two historic flights, which are should have taken place at nineth and twelveth of June. The same querry seven days later showed an additional flight on eleventh of June and also an different operational carrier (Lufthansa City Line)
This is a limitation on how the automatic codeshare lookup is performed. Flight DLH1035 was occasionally operated under the City Line name (CLH1035). When the codeshare was active a search for DLH1035 would return both DLH1035 and CLH1035 results.
Whether or not a codeshare may be active can be anticipated by using AirlineFlightSchedules. When looking up a flight it will return multiple results that refer back to a common “actual_ident” that as the operating carrier in the codeshare relationship. The related entries will have the same origin/destination/departure time. However, this specific arrangement hasn’t occurred since June.
