OK, this is different here:
When I click on the ellipses that open the menu with the "Add Reminder..." item, and the menu is open, then the keyboard shortcut works. In no other situation the shortcut works. In particular not when the focus is on the text area of the note
My reminder Dialogue is different form yours. I get a "Reminder added" box and then I have to do yet another click to add a date (another UI error!)