All PartOperations not added in the current session appear to have a TriangleCount value of 0. I opened a large game with thousands of PartOperations and searched TriangleCount ~= 0
in the Explorer filter, and no results appeared. I also tried this with the Next Gen Explorer to no avail.
I created a union out of 2 parts here and the TriangleCount was 20 in the properties.
After closing the place and re-opening it (another user had the place open so the Team Create server was still active), the TriangleCount showed as 0.
Expected behavior
I expect all PartOperations to show a correct TriangleCount even after re-opening the place.