I guess I should have done a search... there's
this thread that starts with asking about the same problem, but then seems to go slightly off-topic about audio playing back differently to a previously recorded midi driver track, which is not the same problem.
This simply seems to be a difference in how MIDI notes are recorded (time-stamped) and nothing to do with audi playback, which as far as I can tell is dead on in-synch with the midi notes, which is obviously how I detected the problem.
I'm searching the online docs for configuration possibilities, but if someone out there happens to know the right place to look, I'd be grateful...