While it seems odd that it would be specific to Studio, since Studio and Producer have the same audio engine and user interface in general, it's certainly not impossible. In particular, how Pro Channel enters the picture in Producer might well mean some differences in plug-in chain logic, and perhaps even some fixes that were made in the context of behavior with Pro Channel that had side effects if Pro Channel wasn't in the picture. However, I wouldn't be so quick to jump to the conclusion that this is happening only in Studio, unless you've tried X2 Producer on your systems and verified that it does not have the same problem on the same system/configuration/setup.
As it stands right now, if this thread is the totality of the sample size for testing, we only have two systems having the problem on Studio and one not having the problem on Producer, and there are vast differences in my system that is not having the problem and your desktop rig, and I'd guess also your laptop rig, though you don't mention much about that configuration, only that it also has a Firewire audio interface like your desktop rig. Besides the Firewire versus PCI interface differences (not to mention different brands and drivers), and, of course, the Studio/Producer differences, there are graphics card differences (mine is pretty darn basic compared to what many people have these days, so likely does a lot less at the hardware level), CPU differences, system setup differences (probably), SONAR configuration tuning differences (perhaps), etc.
It would be helpful if some others could also test this on both Studio and Producer (odds are no one will be able to do both, at least outside of Cakewalk itself, but even having multiple attempts from different people could show whether there is indeed a consistent issue on one side of the fence and not on the other). If it is consistent on Studio, and it is consistently not an issue on Producer, then that confirms the Studio-only theory. But, if others find it on Producer, or others do not find it on studio, it would point to something system-specific. That still doesn't mean there's anything wrong with your systems, but it would narrow the focus somewhat to trying to figure out what is different, but consistent between your two systems, that might be coming into play.
Rick