I think there must be a bug in Jsitemap somewhere.

I got the error message from google that 404 file is not found.
After checking the links, i found that if the web address are written like this:

example.com/test

There will not be provided an 404 error file

If i write the address like this:

example.com/index.php/test

Then the 404 file will be shown.

And i believe that no matter how you mispell the address, the error file should be found, - not only if the index.php are in the address line.

Jan