A json_lines expects each line to contain an entire JSON object, so that's not going to work if your object is pretty-printed over multiple lines.
You need to use a multiline codec on the input. If you want to consume the entire file as a single event then you can use a pattern that never matches. I use
codec => multiline { pattern => "^Spalanzani" negate => true what => previous auto_flush_interval => 1 multiline_tag => "" }
If you have multiple JSON objects in a file you will need to work out the appropriate regexp for yourself.