Windows 10: Be Afraid, Be Very Afraid

I downloaded a video from youtube and converted to a WMV file.  Under Windows 7,it worked fine.  But copying it to a Windows 10 PC, it always started far into the file regardless of whether I was using Windows Media Player or Groove (the Windows 10 equivalent). Downloading and converting under Windows 10 had the same results.  Utterly mystifying, like all things Windows 10.