Artist is not saved into metadata

[porelli]

$ youtube-dl -xi --audio-format mp3 --metadata-from-title "%(artist)s - %(title)s" --add-metadata --xattrs "https://www.youtube.com/playlist?list=PLFgquLnL59alCl_2TQvOiD5Vgm1hCaGSI"
[youtube:playlist] PLFgquLnL59alCl_2TQvOiD5Vgm1hCaGSI: Downloading webpage
[download] Downloading playlist: Popular Music Videos
[youtube:playlist] PLFgquLnL59alCl_2TQvOiD5Vgm1hCaGSI: Downloading page #1
[youtube:playlist] playlist Popular Music Videos: Downloading 200 videos
[download] Downloading video 1 of 200
[youtube] YQHsXMglC9A: Downloading webpage
[youtube] YQHsXMglC9A: Downloading video info webpage
[youtube] YQHsXMglC9A: Extracting video information
[download] Adele - Hello-YQHsXMglC9A.webm has already been downloaded
[download] 100% of 5.74MiB
[fromtitle] parsed artist: Adele
[fromtitle] parsed title: Hello
[ffmpeg] Adding metadata to 'Adele - Hello-YQHsXMglC9A.webm'
[ffmpeg] Destination: Adele - Hello-YQHsXMglC9A.mp3
Deleting original file Adele - Hello-YQHsXMglC9A.webm (pass -k to keep)
[metadata] Writing metadata to file's xattrs

$ exiftool *mp3
ExifTool Version Number : 10.11
File Name : Adele - Hello-YQHsXMglC9A.mp3
Directory : .
File Size : 4.9 MB
File Modification Date/Time : 2015:10:22 15:23:39+02:00
File Access Date/Time : 2015:10:22 15:23:39+02:00
File Inode Change Date/Time : 2016:03:17 23:29:09+01:00
File Permissions : rw-r--r--
File Type : MP3
File Type Extension : mp3
MIME Type : audio/mpeg
MPEG Audio Version : 1
Audio Layer : 3
Sample Rate : 48000
Channel Mode : Stereo
MS Stereo : Off
Intensity Stereo : Off
Copyright Flag : False
Original Media : False
Emphasis : None
VBR Frames : 15286
VBR Bytes : 5134368
ID3 Size : 49
Title : Hello
Encoder Settings : Lavf56.1.0
Audio Bitrate : 112 kbps
Duration : 0:06:06 (approx)

[dstftw]

Not an youtube-dl issue: #6587 (comment).