and to close this, i have gone through 1000+ midi messages from my controller and stumbled across an error there…on very short notes there sometimes are two note ons sent for the same note and only one note off is sent afterwards. time to debug there! in the meantime i created an abstraction that is less naive in counting held keys, based on an array (with one value for each midinote). find attached a version for pd with array and array sum and a version that should compile in heavy that is slightly more complex.