For example, this problem: 116. Populating Next Right Pointers in Each Node.
Since it is not testable, so I have to directly submit it, and need stdout for debugging.

When I submit it directly from leetcode.com, I could see a Stdout section on the bottom of that page.
While when I submit from command line: leetcode submit....
I could not see stdout..
I debug into your code, in submit.js

core.submitProblem(problem, function(e, results) {
  if (e) return log.fail(e);

    for (let i = 0; i < results.length; ++i) {
        let result = results[i];
        for (var key in result) {
            if (result.hasOwnProperty(key)) {
                console.log(key + " -> " + result[key]);
            }
        }
    }

I only see following output, on stdout

ok -> false
answer -> undefined
runtime -> N/A
state -> Runtime Error
testcase -> '{1,2,3}'
passed -> 0
total -> 0
error -> Exception in thread "main" java.lang.NullPointerException
at Solution.connect(Solution.java:20)
at Driver.main(Driver.java:23)
expected_answer -> undefined
type -> Actual
id -> 159485415

Then why submit directly from leetcode.com could see the stdout section. Are you using the same api ?
I am trying to find the rest post details when submitting code,
In Chrome, I use right click-> inspect -> console, while I see nothing.