Check out this OCA module to see if it solves it for you.  
https://github.com/OCA/purchase-workflow/tree/18.0/procurement_purchase_no_grouping

Here is a copy/paste from the github page about the features:

Go to each product category, and select one of these values in the field "Procured purchase grouping":

• Standard grouping: With this option, procurements will generate purchase orders as always, grouping lines and orders when possible.
• No line grouping: With this value, if there are any open purchase order for the same supplier, it will be reused, but lines won't be merged.
• No order grouping: This option will prevent any kind of grouping.
• <empty>: If you select nothing, default value set up in System settings will be applied.
• Product category grouping: This option groups products in the same purchase order that belongs to the same product category.

System default behaviour can be set up in System settings / Purchase / Procurement Purchase Grouping

Have you found a solution? 

I am also looking for something similar.  

I have a workaround.  I have added a variant ( name ) free text no index. On the SO I can add a description and this will ungroup the lines.   

I want different lines on the same PO. Even if they are the same product. So basically if another like of the same product gets added later to the same PO, I do not want Odoo to group them into a single line.