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.
Not sure how credible that is:
Just confirming that this is only temporary, and related to YouTube's efforts to better respond, review and remove graphic, violative content from YouTube. Thanks for your patience while we work through this.
https://www.reddit.com/r/youtube/comments/b1plj5/sort_by_upload_date_not_working/eiojtjh/
Bug reports for this include:
https://issuetracker.google.com/issues/128673031
https://issuetracker.google.com/issues/128673552
Both have an official comment from Google:
Thank you all for bringing this up here. These specific filters have been currently disabled on both YouTube.com and through the APIs. We should have updates on this soon. Thanks again.
In case you are looking to get the latest videos of a channel:
Alternatively, you can use the channels RSS feed:
https://www.youtube.com/feeds/videos.xml?channel_id={ChannelId}
https://www.youtube.com/feeds/videos.xml?user={User}
This does not replace the full search sorted by date function however.
Thanks YouTube for making me work on a Sunday, all I wanted was to watch the latest videos from my favorite channels. At least I don't have to wait for a fix anymore, good luck to those less lucky!