ffmpeg not copying FRONT_COVER image metadata during conversion

When doing an mp3 → mp3 (or flac → mp3) conversion, -map_metadata can be used to copy metadata from the input file to the output file:

ffmpeg -hide_banner -loglevel warning -nostats -i "${source}" -map_metadata 0 -vn -ar 44100 -b:a 256k -f mp3 "${target}"

However, when I use this, I notice that it doesn't copy all the metadata correctly. Using the tool [eyeD3](https://eyed3.readthedocs.io/en/latest/) , I inspecting the input and output files see this:

$ eyeD3 input.mp3 
input.mp3  [ 4.15 MB ]
--------------------------------------------------------------------------------
Time: 01:46	MPEG1, Layer III	[ 320 kb/s @ 44100 Hz - Stereo ]
--------------------------------------------------------------------------------
ID3 v2.3:
title: Track title
artist: Artist Name
album: Album Name
album artist: Various Artists
composer: Composer Name
recording date: 2019
eyed3.id3:WARNING: Non standard genre name: Soundtracks
track: 17/37		genre: Soundtracks (id None)
disc: 1/1
FRONT_COVER Image: [Size: 86555 bytes] [Type: image/jpeg]
Description: 

PRIV: [Data: 42 bytes]
Owner Id: Google/StoreId
PRIV: [Data: 40 bytes]
Owner Id: Google/StoreLabelCode
--------------------------------------------------------------------------------
$ eyeD3 path/to/output.mp3 
/tmp/test.mp3                                                       [ 3.26 MB ]
--------------------------------------------------------------------------------
Time: 01:46	MPEG1, Layer III	[ 256 kb/s @ 44100 Hz - Stereo ]
--------------------------------------------------------------------------------
ID3 v2.4:
title: Track title
artist: Artist Name
album: Album Name
album artist: Various Artists
composer: Composer Name
recording date: 2019
eyed3.id3:WARNING: Non standard genre name: Soundtracks
track: 17/37		genre: Soundtracks (id None)
disc: 1/1
PRIV: [Data: 40 bytes]
Owner Id: Google/StoreLabelCode
PRIV: [Data: 42 bytes]
Owner Id: Google/StoreId
--------------------------------------------------------------------------------

Specifically, it's not copying the FRONT_COVER image correctly - somehow it's being dropped along the way. **How can I ensure that the FRONT_COVER Image is copied during the conversion process?**