Neither error log nor output are printing out for some test cases

[weebao]

{
    "success": true,
    "message": null,
    "test_results": [
        {
            "expected": "'e'",
            "output": "",
            "passed": false,
            "error": null
        },
        {
            "expected": "'y'",
            "output": "",
            "passed": false,
            "error": null
        },
        {
            "expected": "'a'",
            "output": "a",
            "passed": true,
            "error": null
        },
        {
            "expected": "'a'",
            "output": "a",
            "passed": true,
            "error": null
        },
        {
            "expected": "'d'",
            "output": "",
            "passed": false,
            "error": null
        },
        {
            "expected": "'z'",
            "output": "z",
            "passed": true,
            "error": null
        },
        {
            "expected": "'a'",
            "output": "a",
            "passed": true,
            "error": null
        },
        {
            "expected": "'b'",
            "output": "b",
            "passed": true,
            "error": null
        },
        {
            "expected": "'e'",
            "output": "",
            "passed": false,
            "error": null
        },
        {
            "expected": "'a'",
            "output": "",
            "passed": false,
            "error": null
        }
    ],
    "sample_results": [
        {
            "expected": "'e'",
            "output": "",
            "passed": false,
            "error": null,
            "input_data": "findTheDifference('abcd', 'abcde')"
        },
        {
            "expected": "'y'",
            "output": "",
            "passed": false,
            "error": null,
            "input_data": "findTheDifference('', 'y')"
        },
        {
            "expected": "'a'",
            "output": "a",
            "passed": true,
            "error": null,
            "input_data": "findTheDifference('ae', 'aea')"
        }
    ],
    "summary": {
        "total_tests": 10,
        "passed_tests": 5
    },
    "runtime_analysis": null,
    "deducted_hp": 4,
    "problem_solved": false
}