it’s a DL award … KL flight number, with a notation that it’s operated by German Airways
the “Seat Selection” link on the app says “Oh no! We’re sorry but seat map for this flight is not available. Contact German Airways GmbH …”
the fact that KL wouldn’t sell BKK-AMS-FRA on these flights should have been a clue (but along the same lines, I’m a bit surprised that the 13-hr overnight connection didn’t appear)