--custom-meta make %(invalid_key)s blank. This means

--custom-meta 'album=%(playlist_title)s' will not have an album
  tag for media with playlist_title, as opposed to having it
  set to NA. This is more in line with --add-metadata's
  behavior

youtube_dl/postprocessor/ffmpeg.py

@@ -369,7 +369,7 @@ class FFmpegMetadataPP(FFmpegPostProcessor):
 
         for m in self._downloader.params.get('custommeta'):
             key, val = m.split('=', 1)
-            metadata[key] = val.replace('\\n', '\n') % collections.defaultdict(lambda: 'NA', info)
+            metadata[key] = val.replace('\\n', '\n') % collections.defaultdict(lambda: '', info)
 
         if not metadata:
             self._downloader.to_screen('[ffmpeg] There isn\'t any metadata to add')