MIDI issue

Not sure how to describe this but it’s happened twice on separate projects.

  1. Open 9.5 pro project that has a chord track and midi events on a VST (say piano or keys)
  2. immediately sometimes, after working other times, the pitch of the MIDI changes to a different key even when no changes are made.

I was editing vocals against a piano and after 20 minutes, the MIDI parts, while they don’t appear to have changed, are playing up about 1 semitone.
After saving the project, closing cubase pro 10 and reopening, everything is fine… for awhile