Interesting… MultiChord works for you and the notes will stop when you stop playing keys ? What happens if you play chords that last 1/8 note ? I will update my Zynthian again. I didn’t want to post an issue if it’s not a bug. Thanks.
1/8 at what tempo? 
I tried playing staccato and sustained notes but can’t trigger the issue. Notes should only as long as I press the key. I tried with each preset (default (no chord - pass through), minor triad , major triad). I had Sfizz on layer (MIDI channel) 1 with multichord inserted as MIDI effect. I played notes from a MIDI piano keyboard via 5-pin MIDI input.
Any tempo. It appears to be just related to Harmonica and Squidpipes Soundfonts.
I am using MultiChord right now with Sfizz and a really nice sample of an Ovation Guitar. It works extremely well riban! The chords sound great! I wouldn’t worry about it, that’s why I didn’t create a ticket. I think chords with a harmonica and bagpipes are completely different anyway. Not the same as keyboard or strings. No worries. It’s probably working like it should.