I have this same bug. It always happens to me when I use the autochannel, am playing a note in, and then switch channels on the syntakt. The first track will get bugged and start playing on note on and note release, even using the internal keys. I mentioned it a while ago in the syntakt bugs thread here: SYNTAKT Bugs Thread - #307 by warpigs330 And here: SYNTAKT Bugs Thread - #353 by warpigs330
I believe that the issue is caused by the channel not receiving the proper midi note off command after switching channels and letting go of a key.