I've discovered (by accident) a workaround of sorts for this issue.
I have a project with Oberheim SEM V, Jupiter-8V2 and CS-80 V2 (and some other software synths). I discovered that if I played either the Jupiter-8V2 or CS-80 V2 first and then played the Oberheim SEM V, the Oberheim would actually play properly.
I'm not sure if this is just a peculiarity of this particular project, but I've been following the above routine every time I open the project, and so far, the Oberheim SEM V has worked as it should.
Anyway, I hope this info is of use to people.