To add a note, a way of provoking this is to go to the "Home Screen"???... of the Workstation and the back to the image spanning 2 screens... This seems to be 100% reproducible this way. (Or at least close to in any case)... Not sure if that may relate to it. I suppose this is a bug at this time.
This is disappointing as I've been suffering from this problem since version 7 when I first started using two monitors. It got even worse with version 8, to the extent that I reverted back to 7. And now it seems it's still not been resolved in version 9. Doubly disappointing now that I'm thinking of upgrading to that version. I don't think I'll bother.