Shortly:
Fingerprints are saved on the AcoustID database. The tag “acoustid_id” Picard can save contains just the ID of an AcoustID fingerprint, not the fingerprint itself. I think you’d need to run “Scan” to fetch the IDs from the database.
Just be aware the AcoustID DB is having difficulties for almost 2 weeks already, new submissions are not shown. So if the IDs are not there yet you can’t get them now. Unfortunately not a word from the maintainer.