To see your album art in Rekordbox after a sync, you need to reload tags in Rekordbox. You can do that by right clicking your tracks in Rekordbox.
Reloading tags will take all the track information from your files' ID3 tags, if that is old information, you may need to right click your tracks in Lexicon ➡ Use ➡ Write Tags To File before reloading tags in Rekordbox.
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