It really sounds like extraneous noises and sound are triggering the synth. This is even an issue with guitar and string and pick noise. I must play very cleanly when using a guitar synth. Usually there is a setting to adjust the sensitivity. This can be as simple as a setting choosing Bass or Guitar all the way to exactly what frequencies (ranges) are detected. I forget if the SY-200 has this feature and how sophisticated it is. One thing that can be helpful with guitar is a Noise Gate. This will only let sounds of a certain level through. This is why even though the pitch detection technology has really gotten better, my experience with guitar is using a hexaphonic pickup is best. “Hex” meaning a pickup for each string. An individual pickup for each note of the Accordion is quite another subject.
I know this might not be easy, but if you could post a recording I feel pretty confident I could tell what is happening.
I’m going to post this link to show a technology that exists that may be something to know about for future possibilities with the accordion if you are really interested in converting notes to MIDI and triggering devices and software. I don’t even know if this company in the video can adopt it to accordion. I just thought it might be something you may be interested in checking out. This may be way beyond the scope of what you want to do.