@XCP-ng-JustGreat thanks for the info. I have tried following your instructions for a test Windows 10 VM I have and unfortunately it has not helped the issue at all. I will note however that I haven't had a chance to update XCP-ng for a few months now, so I won't be running the latest version.
One 'issue' I have with your theory is that, in my case at least, it's the USB controller that's bombing, which is causing the mouse to stop working. I expect if USB pass-through was being used, this would also stop working for the same reason.