We've seen a few similar reports, we'll pass yours along too and follow up once we know more.
@TeamYouTube Now I cannot arrange my Watch Later list videos. I can't put whatever video I want in first place, etc. Please fix that! 
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Thanks. Twitter will use this to make your timeline better. UndoUndo
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Following up – this issue has been fixed now, thanks again for your report!
Thanks. Twitter will use this to make your timeline better. UndoUndo
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