Sounds like a bug. I'd think CW would be interested in having a look at your project.
I doubt it has anything to do with drivers, firmware or VST3. More likely, SONAR is either making a mistake when checking the routing, or has failed to detect a routing change.
I believe it only performs the check when something changes that requires recalculating PDC. You may be able to force it to re-evaluate your routing by temporarily re-routing the track to another bus, saving the project, then changing the send back. Restarting SONAR might also be necessary.
What could have happened is that you'd had the track routed to a dead end at some time in the past, and SONAR didn't pick up that you'd changed it.