Without knowing your setup there may be a MIDI input issue causing the problem you are experiencing which can happen if you (the computer I mean) has access to multiple MIDI inputs. This can happen for example if you have a USB keyboard which is also connected via MIDI cables. When a note is pressed the MIDI information is transmitted down both cables which may (or may not) have a bearing on your problem.
I would lean toward it being an input issue as you are the first person, I have heard of that is experiencing this problem.
PS. Do you have any special keyboard settings that may affect what is being transmitted - splitpoints, transpositions etc?
Also - does it mess up ‘normal’ instruments? Trumpets, Piano etc?
I would suggest testing your system by using just the bare minimum.