Ah ha! When I selected just the offending notes, and did a reset overrides on those notes only, it has FIXED the problem.
And guess what? I entered those notes manually - I did NOT record them live.(however, there has been a lot of live MIDI recording in this project - just not in these particular measures)
So, in fact, my problem appears to be very similar to that other thread afterall.
Uploading a project in due course.