为什么聚合操作中$.add返回会是对象？

11月1日补充，以下问题仅出现在JQL中，若在云函数中使用uniCloud.database()获取db执行，则返回正常，与腾讯云开发表现一致。
如题，$.add返回形如：{command:value}，$.subtract返回却是值。（以下测试在JQL调试器中）
实例如下：
test表只有一条数据：

{  
    "id": "5d352be5558b81140fe3c141",  
    "content": "test",  
    "as": []  
}

查询语句如下：

db.collection("test")  
    .aggregate()  
    .addFields({  
        a: $.cond([$.isArray("$as"), $.size("$as"), 0]),  
    })  
    .addFields({  
        n: $.add(["$a", 1])  
    })  
    .end()

结果如下：

[  
    {  
        "_id": "635a3ad4737e2800019fea54",  
        "a": 0,  
        "as": [  
        ],  
        "content": "test",  
        "id": "5d352be5558b81140fe3c141",  
        "n": {  
            "command": 1  
        }  
    }  
]

若是使用$.subtract，则为：

[  
    {  
        "_id": "635a3ad4737e2800019fea54",  
        "a": 0,  
        "as": [  
        ],  
        "content": "test",  
        "id": "5d352be5558b81140fe3c141",  
        "n": -1  
    }  
]

除了上述问题外，这个查询语句在客户端或云函数中使用uniCloud.databseForJQL获取db调用时会报错如下：

InternalServerError: errCode: InternalServerError | errMsg: Command failed with error 40180 (Location40180): 'Invalid $addFields :: caused by :: an empty object is not a valid value. Found empty object at path n' on server 192.168.146.109:3717. The full response is { "operationTime" : { "$timestamp" : { "t" : 1666885450, "i" : 16 } }, "ok" : 0.0, "errmsg" : "Invalid $addFields :: caused by :: an empty object is not a valid value. Found empty object at path n", "code" : 40180, "codeName" : "Location40180", "$clusterTime" : { "clusterTime" : { "$timestamp" : { "t" : 1666885450, "i" : 16 } }, "signature" : { "hash" : { "$binary" : "2ggBcFxRNhaYiNAPEKLVvLvBHn4=", "$type" : "00" }, "keyId" : { "$numberLong" : "7124964235662065714" } } } }