You can use below method to update product attribute value.
$product = $this->productRepository->getById($productId);
Also, if you want $objectManager method to create a script. You can do it by following below code snippet.
Note that calling Magento\Framework\Model\AbstractModel::save() directly has been deprecated since Magento 2.1.10 (see https://github.com/magento/magento2/blob/2.1.10/lib/internal/Magento/Framework/Model/AbstractModel.php#L629).
I have tested saving product using Magento\Catalog\Model\ProductRepository::save($product) and your issue did not happen.
You define what forms appear in as part of setup. Have you missed this part?
If you're asking for a "quick" way to have the stores use the default/website values again, then you'd be looking for a way to actually just delete the data at that store scope; the absence of store scope data implies that it should fall back to the default/website values.
Essentially you'd be wanting to remove data from tables like such (please, for the ...
You can use below MySQL query to get the required records from the database
`e`.`sku` as SKU , `e`.`entity_id` as 'Entity ID', `attribute_table`.`attribute_id` as 'Attribute ID', `attribute_table`.`value` as 'Value'
`catalog_product_entity` AS `e`
`catalog_product_entity_varchar` AS `attribute_table` ON (`...