I am uploading 1920x1080, and 1280x720 mp4 to my CC library and they are being resized to 500x500 and thumbnails are also being removed in the online viewer. Also when i share these videos they are smaller and don't provide a thumbnail.
This all started happening a week ago, before then everything worked as expected.
Try and upload a video to your library and see if the same thing happens to you.
Someone on the forum said its a possible metadata issue, which seems like an easy fix to me, but i also don't know.