Hi Alvin, this wouldn’t work because you are referring to the Milestone Status whereas you want to show the Project Status in the dropdown.
Ideally, you can do a search for All Projects, where Milestone contains the Project ID, and then display the unique Status in the dropdown

A better way would be to establish a relationship between your Milestone and Project table, such that a Milestone table contains the field to your Project table. In that case, the expression will be simplified to RepeatingGroup Milestone: each item’s Project: each item’s Project status: unique elements