You observe that the Unit of Measure or UoM is selected incorrectly when you are adding a Product from Product List in an Opportunity.
Reproducing the Issue
Go to Sales Work center
Navigate to Opportunities View
Select Show All and choose any random Opportunity
Navigate to Products Facet
Select Add --> From Product List
Select Product List ABC and Add Product DEF
You expect the UoM selected to be the proposed UoM 'X', but system is taking it as 'Y'
All the products selected to be added have a sales specific UoM maintained as Y for the Sales Org.
In case you want to have UoM as X, then the same should be maintained as the UoM for the Sales Org as well.
KBA , SRD-CC , Cross Components , How To