Please follow the mentioned new portal item. I am marking this one as "declined" just because of the duplication.
Thank you for reporting this bug. I have logged it in our feedback portal and will try to have it fixed for the Service Pack later next month.
Diagrams: SimplifiedMode of DiagramRibbon breaks the functionality of most of the Tools and Settings buttons, numerics, combos.
I have also updated your telerik account points as a thanks for your cooperation.