I have an Opportunity Product/Opportunity Line Item lookup-relationship on a custom object in my development sandbox. The custom object also has a lookup relationship to an Opportunity. When I try to select an Opportunity Product relationship, I only see a subset of the available Opportunity Products in the select options. Newly-created Opportunity Products which have not been edited are not visible. If I navigate back to the newly-created Opportunity Product which I want to establish a relationship to, change any field, save, and then return to my custom object, that Opportunity Product is now visible. The relationship field on the custom object is editable for all profiles and visible to all on the page layout.
What appears to be happening is that Opportunity Products are invisible to lookup relationships if the created and last modified datetime stamps are ~equal.
As I don't want to have to advise users to edit all of their Opportunity Products, I'm hoping there's some other solution.