Good question! The flag --full-remote-verification triggers that all files are downloaded and the contents are checked.
I can see from the output that all files are downloaded, so the flag is activated correctly.
The error message indicates Extra contents, which means that it has more data than it is supposed to. This means that the local database and the remote files are out of sync. Most likely cause is that Duplicati has flagged the contents of the files to be deleted, thus it finds no contents for the files, but the files are not empty.
I can only guess that this deletion has happened after the backup, but before the test ?