In a grouped product, associated simple products default quantity not saving. The values are saved in database in catalog_product_link_attribute_decimal table, but not showing in admin (blank input on product save) or frontend.
I found the problem. The shop owner created a product attribute with 'qty' attribute_code.
getQty() method was calling that attribute value instead of default magento qty.
After deleting that attribute the default qty values for simple products were saved in admin and appear on frontend