This issue has been reported for Cubase and EastWest PLAY system (EWQL). It may also occur with other plug-ins, sound libraries or DAWs.
When running PLAY inside one or more Drones in Cubase, you may experience hanging (dangling) notes. That is, notes continue to sustain after playback stopped. The issue does not occur in the Synfire Audio Engine. It does not seem to have anything to do with Synfire.
This is a know issue, keeping users struggling for many years, as a quick Google search (or this search) suggest. We found a suggested solution in the Steinberg user forum. Please try that solution before contacting support:
Please have a look at your MIDI Input setting in Cubase. If it is set to All MIDI Inputs, change it to your main input only (e.g. your master keyboard or control surface). This was reported to do the trick.
The suggested solution indicates a loopback problem, that is, MIDI data sent by Cubase goes to some external hardware or Loopback MIDI Driver and then back into Cubase, where it is handled like live keyboard input. This feedback causes a number of issues, hanging notes being one of them.
Let us know with a comment, if this helped.