I have been so frustrated with these CSU so I wrote a long mail to Diamond service. Stating all reservations that has worked and not worked in the past (even Rewards bookings).

Their reply:

“The issue you’ve encountered is linked to a system limitation with certain rate codes. CSUs are designed to apply only to qualifying rates, but occasionally the system incorrectly blocks them - even on Reward Night bookings. Our technical team is aware of this and is working to improve consistency so that CSUs can be applied more reliably across all eligible reservations. “