I wrote a basic tool to upload media to Google Photos via the official API: https://developers.google.com/photos/library/guides/upload-media
Everything in the described flow works, I'm able to upload media and create the media item in Google Photos.
I receive a photoURL containing the processing video. However, a short while after the video processes the URL starts returning a 404 and the upload is gone from the Photos Library.
This doesn't happen with all videos - An arbitrary MP4 and MOV tested work just fine, but various media from a GoPro camera (MP4 in 4K) fails consistently.