From polkit manual page:

The log() method writes the given message to the system logger prefixed with
the JavaScript filename and line number. Log entries are emitted using the
LOG_AUTHPRIV flag meaning that the log entries usually ends up in the file
/var/log/secure. The log() method is usually only used when debugging rules.
The Action and Subject types has suitable toString() methods defined for easy
logging, for example,

None of this works with duktape engine which just dumps whatever it got as
string, so attempt to print action or subject or anything non-string results in
error. And in any case, no file name/line number is added even for string
content.