I don't have an SMS one as yet, I do have little scales genmdm for megadrive (the catskull one) , Genajam was another hardware device that made genmdm simple to use without max or a pc etc. It's been a couple years but on the megadrive I think the next note wouldnt play unless the first one was released or it might have been like you described, either way it was not really able to played like you would expect from a synth. I was able to change the code in Genajam (as genmdm is not open source) so you can hold down one note and press another note, it will switch to the new note then if you let go of the new note, it knows the other note still being held as it's not recieved a note off midi packet. so it starts playing that one. I tried not to use much RAM so it only handles like 6 notes at a time I think and isn't perfect (I should have made it keep track of 128 notes of whats on and whats off) but then you get into note prioritys , high note, low note, last note etc.
It did make it enjoyable to play on a keyboard. I am NOT very good at C++, I am more this is bugging me so much I will figure out how to fix it as best I can. I had a quick look at the midi note on note off in the SMS one and it's a bit more intimidating code.
Anyway I will try and get one of the SMS setups going this year and I don't want to speak for little scale at all but appreciate its now on github and the codes updated to complile on modern arduino IDE.
are you using it with an FM equipped sms?
, it's dying for a sequencer. Currently sample food.