Disable multiple tag names for free-form tag formats

I don't know if this comes from beets or if it's me configuring ncmpcpp poorly.
I got this line in .ncmpcpp/config :
song_columns_list_format = "(5)[red]{n}(5f)[green]{l} (25)[cyan]{a} (40)[]{t|f} (25)[red]{b}"
This is the default way to display the songs in the column format in ncmpcpp except that I added the track number: (5)[red]{n}
The issue is that tracks are displayed that way :
If I go into the ncmpcpp tag editor :
Of course all those tracks were imported with beets,
it seems like a strange format. Is that a bug from beets mistagging the tracks or a ncmcpp rendering issue ?

Start by reproducing the reported configuration with beets-imported tracks and compare the stored tags with how ncmpcpp displays them. The issue does not name a beets file or test; trace the free-form tag formatting path and determine whether beets writes multiple tag names or ncmpcpp renders them incorrectly. Done means the source of the duplicated names is identified and the requested behavior for free-form tag formats is specified.

Written by the indexing model from the issue text.