Last big heavy bike I rode before the K1600s was about 10 years ago, on a VTX 1800R. That bike was planted solid no matter what the speed, or surrounding traffic. I've even had other riders comment that they saw some bikes moving around as we passed big rigs, but I looked like a freeze frame.
Since then I've owned a GTL, a GT, and now a B. The whole K1600 series is not as planted as that VTX. We all experience some amount of movement at high speeds. Invariably, different people will report this differently on different models, etc. I always read a rider's report with a grain of salt. How shall I put this? There are different levels of expectations and sensitivities amongst riders.
That said, on the B, I have too experienced some level of movement at speed. A little more than the other models. That doesn't mean hanging on for dear life. Far from it. My bigger concern at first was getting the right windshield so my head wasn't getting slapped around. And that is solved now. I've also replaced the stock BattleAx with the RS4, which has made a difference all around.
It's a matter of perspective. I'm calling it a "little movement". Others are calling it "instability". That's a gross overstatement. Have we not all ridden unstable bikes in our lives? This is not one of them.