I never used the QWERTY Keyboard, but I gave your steps a try and had the stuck note bug. I recommend reporting it officially.
https://cherryaudio.kayako.com/
Pressing and releasing note that is stuck from a usb keyboard, turns off the stuck note.
Other observations:
(1) You didn't mention turning the Sustain on, but I am guessing you had it on (maybe) because one of your steps is "press some notes on the QWERTY keyboard and release them."
(2) Sometimes when turning Sustain on or off and pressing the Reset button, I did not get a note being stuck when I disabled the Arp.
(3) Sometimes for no apparent reason, the arp goes into double time. Sometimes when this happens, changing the Sustain and Reset states as well as notes previously selected to arpeggiate will knock the double time off. Once that happens, there isn't a stuck note after disabling the Arp. (This seems like what you describe as "Spamming the original notes played notes on the QWERTY Keyboard will eventually kill the stuck note.")
FWIW I don't use an MPE Keyboard, so under Settings I have MPE Mode Disabled.
Manual References:
Sustain- The Sustain button mimics the functionality of a standard sustain pedal. Click the [TAB] key to engage sustain, or [SHIFT]+[TAB] to lock it. The Sustain button can also be engaged by mouse clicking it.
Reset- Initializes all MTK parameters including keyboard range and control parameters.
IMPORTANT: If MPE mode is enabled, you must use a MPE controller set to”Lower Mode” to achieve full MIDI control. If you're using a non-MPE MIDIController, or the QWERTY Musical Typing keyboard, only one mono voice will trigger