Differently dataset object saved than it is loaded.
### Describe the bug

Differently sized object is saved than it is loaded.

### Steps to reproduce the bug

Hi, I save dataset in a following way:
```
        dataset = load_dataset("json",
                               data_files={
                                   "train": os.path.join(input_folder, f"{task_meta_type}_{task_type}_train.jsonl"),
                                   "test": os.path.join(input_folder, f"{task_meta_type}_{task_type}_test.jsonl")})
        print(os.path.join(output_folder, f"{task_meta_type}_{task_type}"))
        print(f"Length of train dataset: {len(dataset['train'])}")
        print(f"Length of test dataset: {len(dataset['test'])}")
        dataset.save_to_disk(os.path.join(output_folder, f"{task_meta_type}_{task_type}"))
```

this yields output
```
.data/hf_dataset/propaganda_zanr
Length of train dataset: 7642
Length of test dataset: 1000
```

Everything looks fine.

Then I load the dataset
```python
from datasets import load_dataset

dataset_path = ".data/hf_dataset/propaganda_zanr"
dataset = load_dataset(dataset_path)

print(f"Length of train dataset: {len(dataset['train'])}")
print(f"Length of test dataset: {len(dataset['test'])}")

```

this prints
```
Generating train split: 1 examples [00:00, 72.10 examples/s]
Generating test split: 1 examples [00:00, 100.69 examples/s]
Length of train dataset: 1
Length of test dataset: 1
```

I dont' understand :(


### Expected behavior

same object is loaded

### Environment info

datasets==2.16.1