I suspect a lot depends on the ability and capabilities of the player. For example, I have the NU1 firmware update (I believe you gave it to me), but decided to not install it after discussing the issue with my piano teacher: initially I occasionally experienced the "loud note" issue, but then my teacher explained that my technique was incorrect (I had just started with him). After some time in which I am doing lots of things with him (including technique), I am never experiencing the problem anymore in regular playing. I know the problem is still there because I can trigger it at will if I decide to do so, but I simply don't play "that way" anymore (despite my main instrument now being a grand piano).
Now, as @CyberGene explained, problem is purely hardware and the fix is not a fix, but a purely software workaround (I am pretty sure there is no AI involved, just time measurements, but that's irrelevant). I suspect @eddiepiano has a great technique, since repeating very fast multiple notes @ fff level is not commonly done, and I suspect Yamaha did not get this workaround tested at that level (or if they did, they thought it was a virtuoso thing unlikely to be in the target audience).
eddiepiano I can't believe it, how can Yamaha still produce and sell these pianos with this systematic error due to their non-sufficient one sensor system? And I can't believe how nobody has noticed this until now? It's less noticeable than a sudden out of place loud note of course, so for Yamaha this may seem to be a better "solution" than the loud note issue.
I think this behavior is more closely related to how the upright action works: if you don't fully release the key, you may easily get a very quiet or even silent second/third note. So incorrectly played fast repetitions on the NU1(x) and on an upright are now behaving identically. Of course if you are virtuoso as I am speculating, you know that very well and perhaps you are fully releasing the keys, but if you aren't/don't give it a try on a number of upright pianos and see how they behave.
That said, Yamaha could have done better. Changing the sensors would have required quite a change of the instrument, so while I agree with your sentiment, I can't really blame them from not doing. Digital pianos have all sort of issues that you can never expect a perfect one. But, Yamaha could have made lemonade out of the lemons, by making a settings for this "fix" to be implemented as different "modes". I proposed "practice" as the name for the fix "off", so you'd notice your poor technique with a loud note, and a "performance" name for the fix "on". Not sure if @eddiepiano would have tolerated the loud note issue (like I do), but at least he could have had another choice rather than simply returning the instrument.
As for cheaper alternatives, if you have time/stamina, you could engage in the Build your own hybrid piano which is the natural next step of the Cybrid.