When I stayed at hotels that didn't even present a receipt (e.g. PH DC & PH Chicago) I would leave additional cash tip at the table as the service was usually very good. However, getting a buffet at the HR Irvine there is little, if any, service that warrants tip, yet it is routinely pointed out as the poster child for saying tip is not included. The certificates they hand out even say it on them.
If the GP policy is to include tip then the properties should include it. If GP wants to change the policy that is their right to do so. But, it is has been repeatedly stated on the GP website and here and again in this OP that tip is included. Unless GP changes the policy it is something to fuss over.
My working theory is that the hotel gets reimbursed by GP for the diamond breakfast and some hotels keep the tip as a "handling fee" instead of giving it to the staff.
With all that said, I'm no longer a Diamond () so this doesn't affect me this year.