@mariosasko, I guess my problem of concatenation between train and validation when using load_dataset() seems to be a normal behavior if no dataset loading script is created in the files on the HF dataset repository (check this post):
However, this information is not given in the HF doc Upload from Python about how to upload a datasets.DatasetDict on the Hugging Face Hub in Python.
The HF doc has to be updated or the DatasetDict.push_to_hub() has to be modified?
cc @lhoestq