Sorry to hear that the preset dropdown isn't funcitoning correctly, I certainly understand that using the arrows would get tedious!
What version of Cubase are you running? The early releases of Cubase 8 on Windows had some bugs that caused this behavior in many different plugins, so I'd recommend updating to the most recent version of Cubase 8 if you can.
Additionally, you could try turning the "Always on Top" setting off, in the Cubase plug-in preferences.
Let me know if you have further issues.