For some reason, when trying to use get_fields($variation_id) later (for example on front-end), you won’t be able to get the saved fields.
This is quiet weird as update_field use to store fields seems to do the job the right way. Custom fields are saved in postmeta table : both field_name and _field_name are saves, the first with field value, the second with field key. Furthermore, these fields are restored the right way when editing product variations.
For now, I have to use get_field('field_name', $variation_id) to get data.
