Make sure you have Multi-Step Routes turned on.
Edit the Buy Route:
The default setting for Propagation of Procurement Group is Leave Empty. Change it to Propagate:
You will now get a new draft Purchase Order each time, instead of Odoo leveraging an unconfirmed Purchase Order by adding any subsequent products onto it.
Thank you Ray!
I wanted to point out that I implemented this on my Odoo 17 EE setup and while it worked, there was an issue that it introduced.
For some reason, when a PO is created from a Sale Order using routes, the receipt for the PO was showing in the deliveries area on the SO. In the portal area, customers could see this delivery as well, it showed as if it was a return. It also allowed customers to view the receipt document which exposed our PO# and vendor.
If anyone else uses this way of preventing PO from merging, I would test to make sure this doesn't happen on your environment.
I ended up using this OCA module instead, and it fixed the issue: https://odoo-community.org/shop/procurement-purchase-no-grouping-1900#attr=943127
