The diff uses the snapshot logic to find objects to drop. I committed a change to the extension to make foreign key snapshots a no-op. It fixes the dropAll but it makes diffs between redshift and databases that do support foreign keys think that redshift has no FKs. Probably all we can do until we can figure out how to pull the foreign key info in redshift.
Can you try the code in liquibase-redshift master?
Nathan
Nathan