I was struggeling with this problem for over a year now, getting very paranoid and saved every two minutes. Very often when I forgot to save my project and was i.e. writing lyrics while playing the song, - just when I got back to the project the computer was crashed, - but as you pointed out the music still playing. It was also exactly the same, that nothing could be done except stop the process in task manager, but then wait for ten minutes until Nuendo was really out of the system. Other solution was to hard reset the computer and boot up again. NOT what I would call a workflow.
After app. 18 months of configuring my system, throwing out my old graphics card, going deep into system settings to get rid of background processes and so on and so on, the problem still RANDOMLY occurs. Means, it happens if I play a super simple rompler, but also super complex Kontakt arrangements, - or by times also not for two days. Anyway, I had to come up with a solution to keep working.
After making sure that it is not related to plugins or Kontakt which I checked in Ableton and Presonus Studio One.
I had to decide to switch to Studio One.
It hurts, - for many years I did all projects on Cubase / Nuendo and switching the DAW is a long way, especially if you have to continue a lot of projects written on your old DAW. At this point, I gotta say it was my only solution.
In two years, when I get a new computer, I will give it maybe a go again, but for now Studio One just gives me a secure feeling of trying and enjoying what I like most, without getting completely paranoid and frustrated.
Did you find a solution for your problem at all?
How could you fix it? A new computer?