You were downloading DASH files (m4v and m4a), not flv file, from you log:
The flv is just initial file extension, it will be changed when the download starts.
Quote
Downloading media : YouTubeMedia{itag=266, container=MP4, videoQuality=2160, videoEncoding=H264, dashType=VIDEO, frameRate=30, audioEncoding=None, audioBitrate=-1}
Deleting partial file C:\temp2\Stunning Aurora Borealis from Space in Ultra-High Definition (4K).m4v.8774240858708599251.part
Downloading media : YouTubeMedia{itag=140, container=MP4, videoQuality=-1, videoEncoding=NONE, dashType=AUDIO, frameRate=-1, audioEncoding=AAC, audioBitrate=128}
Deleting partial file C:\temp2\Stunning Aurora Borealis from Space in Ultra-High Definition (4K).m4a.8304572205193927458.part
The flv is just initial file extension, it will be changed when the download starts.