I'm not surprised with the response. You might try a different CSR and see if they are willing to break down your folio to the day you made Diamond, but I wouldn't expect it.

In hindsight, it would have been good to flag your 60th night by checking out and going to a different hotel for the night and then back to where you were. Then there would be no question of getting the right bonus points.

Since you don't get Diamond status until after your stay posts and not just at 60 nights, there was no way for the property to know you have status to post bonus points. Again, if you ask real nicely another CSR may make the effort.

Good luck.