You wrote that you experienced this behaviour even after reinstalling Cubase. I guess you propably have other Cubase versions on your Mac as you have worked with Cubase since version 3. It might be that there are still user preferences on your system that are flawed. Take a look at this thread, the latest approach aims at corrupted preference files:
So, my suggestion would be to search for all user preferences and temporaily rename the folders before restarting. But that is just a guess to rule out all potential triggers for that annoying behaviour… I understand that it is impossible to worh with a bug like this!