I think I have found a bug with the pro-q3 that was driving me nuts for an hour or so today until I figured it out.
When using the .vst version of the plugin, I was experiencing weird behaviour when applied on mono audio tracks. I was experiencing a volume boost amongst other things, and I also discovered I wasn't able to turn down the volume output knob - only turn it up. Example: I would turn the volume down to minus infinity, and I would still be hearing the dry signal at normal volume. My conclusion is that it seems like the plugin is adding the dry unprocessed signal to the dry.
This wasn't happening on stereo tracks, and I resolved it by switching to the vst3 version of the plugin which works normally on mono tracks.
Pretty weird bug. I hope this can be fixed to save some other users a lot of confusion. I'd also like to hear if other cubase users can replicate this behaviour on their systems. I'm running cubase 10 pro on osx.