I'm experiencing this myself. I received my KLII 88 a few days ago, and had enough dropped notes/Key Offs so as to make it unplayable, in addition to the device sometimes being unable to even be enumerated on the USB bus. I updated the firmware to 1.2.4, and while it's much improved, the dropped notes/Key Offs are still noticeably present. I also switched from USB MIDI to running 5-pin DIN cables to my MOTU M4 interface, which improved things further, but again still did not completely eliminate the problem. I don't experience a single dropped event when putting my 32 year-old Yamaha SY-77 through the M4 over the same cables, so I'm reasonably sure it's an Arturia issue. When using the KeyLab II, running Reaper and FLStudio doesn't make a difference - events are missed in both packages with equal frequency. For reference, I'm running on a 32 GB system under Windows 10. All USB cables have been demonstrated to work fine with other devices, and USBView shows information that looks good.
Edit: USB is going in through a port on the machine's motherboard, not through a hub.
It's been a pretty disappointing experience that's kind of soured me on the idea of any further Arturia hardware purchases. As a professional embedded firmware coder myself, my initial thought was that perhaps it was a USB issue, but given that there are still lost events over the 5-pin jacks going to a known working USB MIDI interface, I'm inclined to think there are still issues within the KeyLab II firmware itself. It's been close to a year since the last firmware update - does anyone know if there are there any plans for another anytime soon? If not, I may have to consider returning the instrument, because I can't just eat a thousand bucks for a device that has doesn't work reliably. I think the only troubleshooting step I have left is to put the device on a stable bench power supply (instead of USB) and determine if the lost messages still occur over the 5-pin ports.
