`convert` fails to embed or copy album art to converted files

### Problem

Beets fails to embed or copy album art to converted files, both on import (using `auto_keep`) and manually. Non-converted files get imported correctly with images embedded in each file and a cover.png or cover.jpg in every album folder.

### Setup

* OS: Debian 12
* Python version: 3.12.11
* beets version: 2.3.1
* Running in Docker using linuxserver/docker-beets v2.3.1-ls281
* Turning off plugins made problem go away (yes/no): no

My configuration (output of `beet config`) is:

```yaml
plugins: fetchart embedart convert scrub replaygain lastgenre chroma web
directory: /music
library: /config/musiclibrary.blb
art_filename: cover
threaded: yes
original_date: no
per_disc_numbering: no

convert:
auto_keep: yes
format: opus
formats:
opus: ffmpeg -i $source -y -vn -acodec libopus -ab 192k $dest
threads: 4
dest: /music-lossy
copy_album_art: yes
never_convert_lossy_files: yes
embed: yes
paths:
default: $albumartist/$album%aunique{albumartist album,year,()}/$track - $title
singleton: Non-Album/$artist - $title
comp: Compilations/$album%aunique{albumartist album,year,()}/$track - $title
import:
write: yes
copy: yes
move: no
resume: ask
incremental: yes
quiet_fallback: skip
timid: no
log: /config/beet.log
language: en

match:
preferred:
countries: ['US', 'XW', 'GB|UK']
media: ['Digital Media|File', 'CD']

lastgenre:
auto: yes
source: album

embedart:
auto: yes

fetchart:
auto: yes

replaygain:
auto: no
threads: 3
backend: ffmpeg

scrub:
auto: yes

replace:
'^\.': _
'[\x00-\x1f]': _
'[<>:"\?\*\|]': _
'[\xE8-\xEB]': e
'[\xEC-\xEF]': i
'[\xE2-\xE6]': a
'[\xF2-\xF6]': o
'[\xF8]': o
'\.$': _
'\s+$': ''
```