The sorting is not working as expected since a few hours.
https://www.googleapis.com/youtube/v3/search?order=date&part=snippet&channelId=UC_x5XG1OV2P6uZZ5FSM
Reports in Google's forum were commented by a Google employee: https://support.google.com/youtube/thread/2494861?msgid=2520468
YouTube is aware the search/sorting functions aren't working as expected – this is temporary and part of our efforts to better respond, review and remove graphic, violative content from YouTube. Thanks for your patience while we work through this. Will update this thread when these features are working normally again, feel free to subscribe for updates.