Cannot select a geometry entity from a part created by merging instances

[dork_matter]

I am stuck on defining the sets of my part named “domain” that is the result of a Merge Instances of (8) instances in my assembly. I cannot replace the viewport contents with the air entity with Cells as the type to be replaced. I am unable to select just the air geometry. I am able to for the other (7) entities. I have even tried the “Select Entity Closest to the Screen” from the Selections Toolbar after changing to a view that placed the air entity closest to the screen. The near face of all entities are constrained face-to-face with 0 offset. I have preselection highlighting enabled. For the other entities, I am able to select within the part and the entities are outlined red. When I hover over an entity it is selected and outlined orange. When I try the same procedure with the air entity I cannot select it individually so the entire part is outlined orange.

Can anyone provide any tips?