Does anyone know of a reason why Opportunity Product Line item fields that are editable in Classic (1) are locked down in Lighting UX when you're in the Edit All mode? 

The "locked" fields are custom Picklist fields on the Opportunity Product (Line Item) object, with a field dependency, both the Controlling and Controlled fields are locked (2).    

All of the fields can be edited normally if you edit the Opportunity Product Detail record (3)