When you open a product from a Sales Order, Purchase Order, Bill of Materials, etc (pretty much anything that you "ADD ITEM" to) you are referencing product.product model records. 

When you open the PRODUCTS Menu you see product.template model records. 

Note: Even without configuring Odoo to use Product Variants (product.product), there is one Variant per Template (product.template)

It isn't always straightforward when customizing the "product" form because there are two models. We also sometimes add fields via a view which has the model set to product.product and sometimes set to product.template, even though the fields would apply to the "other" model. 

If you turn on Variants in Configuration --> Settings (you can turn it back off again later) you can see the Product Variants Menu which will show you the product.product views - and you can verify if adding your fields to THAT form has them show up on the Sales Order.