All database names (GUI or CLI) are assigned as random unless you say which dbpath you want.
GUI remembers DB per-job in Duplicati-server.sqlite database. CLI remembers it in dbconfig.json.
Future CLI runs find it again based on the destination. When using CLI for GUI, pass GUI dbpath.
Using the Command line tools from within the Graphical User Interface does this for you, as does
Export As Command-line if you prefer to do your modification from that instead (to use a true CLI).
True CLI runs independently from GUI, but can be sort of connected to GUI through the database.
Almost certainly, if backup was GUI (if not, say so). You can also see its path in database screen.
About → Show log → Live → Warning would probably name them. Kind of awkward, admittedly…
The PURGE-BROKEN-FILES command (and list-broken-files) seems to notice dblocks best.
Although you got a message on at least one dblock, it would be good to see how dindex are now.