If you check the documentation for Video.insert you will find the following at the top of the page. This is a new policy that is recently beginning to be enforced.
Until your application has been verified all videos you upload will be set to private. You need to go though the audit first then you will be able to upload public videos.
Note once your application has been verified this will not automatically set all existing previously uploaded videos to public you will need to do that yourself.
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