Wrong `is_labeled` attribute

Hi,

while downloading tasks with the Python SDK (`project.get_labeled_tasks()`), we found that not all labeled tasks are retrieved. After some investigation and building the difference between all tasks (`set(project.get_tasks()) - set(project.get_labeled_tasks())`) some tasks showed annotations, but the `is_labeled` attribute is still `False`:

~~~
{
"id": 101759,
"predictions": [],
"annotations": [
{
"id": 3631,
"created_username": "xxx, 36",
"created_ago": "3\u00a0weeks, 6\u00a0days",
"completed_by": 36,
"result": [
{
"id": "ddGsmMEoOO",
"type": "choices",
"value": {
"choices": [
"good"
]
},
"origin": "manual",
"to_name": "image",
"from_name": "choice"
}
],
"was_cancelled": false,
"ground_truth": false,
"created_at": "2023-03-17T13:42:32.167708Z",
"updated_at": "2023-03-17T13:42:32.167713Z",
"lead_time": null,
"task": 101759,
"parent_prediction": null,
"parent_annotation": 614
}
],
...
"total_annotations": 1,
...
"annotations_results": "[{id:ddGsmMEoOO, type:choices, value:{choices:[good]}, origin:manual, to_name:image, from_name:choice}]",
...
"is_labeled": false,
...
}
~~~

________

**Version info**
~~~
{
"release": "1.6.0rc5",
"label-studio-os-package": {
"version": "1.6.0rc5",
"short_version": "1.6",
"latest_version_from_pypi": "1.7.2",
"latest_version_upload_time": "2023-03-20T16:52:30",
"current_version_is_outdated": true
},

"label-studio-os-backend": {
"message": "Bump rc5",
"commit": "b0df5dd19a5400e8d26fb8269e2add87799af055",
"date": "2022-09-27 16:32:57 +0100",
"branch": "HEAD",
"version": "1.6.0+0.gb0df5dd"
},

"label-studio-frontend": {
"message": "fix: DEV-3510: fix video regions w/o label",
"commit": "5d4231c2a8b9703f8c43abdf62948ac892d6f742",
"branch": "master",
"date": "2022/09/27 13:58:00"
},

"dm2": {
"message": "feat: DEV-3373: Enable video preview (#106) * Enable video preview ...",
"commit": "8374cbfeceb5aa9870278fc5371f0585d452c111",
"branch": "master",
"date": "2022-09-27T08:11:03Z"
},

"label-studio-converter": {
"version": "0.0.44"
}
}
~~~