Are you using an Apple Magic Mouse? If so I had exactly the same problem and logged a support request with Steinberg over a month ago. (I’m still waiting to hear back)
I don’t remember when the problem started but Cubase used to work fine and then all of a sudden double clicks stopped working but were fine in all other apps and the OS. I think it was maybe after an OS update that the mouse polling changed and this affected Cubase in some way???
I tried everything I could think of, reinstalling Cubase, reinstalling OS, upgrade to Sierra, upgrade from Cubase 8.5 to 9, changing mouse, re-syncing Bluetooth etc. It was driving me nuts! Then one day I bought a new mouse pad and it got even worse and I realised the problem had been staring me in the face all along…
I got rid of my mouse pad and now just use the mouse direct on the desk surface. Double clicking works all the time for me now in Cubase!!