I can’t reproduce it, probably because it’s a fresh installation of MacOS Sierra and no preferences from earlier Cubase Pro versions have been imported. Unfortunately, every system differs from the other so I would suggest giving a safe start a try: https://www.steinberg.net/nc/en/support/knowledgebase_new/show_details/kb_show/safe-start-mode-resetting-the-cubase-preferences.html
If the double click works perfectly fine while on safe start mode, you will need to reset the program preferences. In order to do so, open the Finder, press the Keys “Cmd + Shift + G” (Or Go to → Folder) and navigate to the user’s library:
~/Library/Preferences
Rename all the “Cubase” folders you find there or move them out of that path to your Documents folder for instance.
Cubase will create a new preferences folder during its next start.
Select an empty project and configure your VST Connections if needed. Close Cubase.
Open a Cubase project from “File → Open project”.
Older preferences that might have taken a lot of time to configure (for example the KeyCommands.xml) can be copied from the newly renamed preferences folder onto the recently created one. Make sure that Cubase is closed before doing this.