David Lai
Heard from them very quickly. In short, they removed this feature for now because of increased CPU usage when sympathetic resonance is generated. On Modern U, you can do this and the resonance is still ringing when pedal is off. But they're afraid that if they kept this feature, users will complain about clicks and pops because of CPU usage.
Right now they decided to remove this feature (sympathetic resonances are not sounding when sustain pedal is down), but they increased the polyphony for the sustain pedal during full and max. They might re-introduce it in the future, but have to progress with care. It's also partly due to engine limitations, so a middle ground has to be taken.
Hope this explanation makes sense.