One last time before I give up: What exactly is the NanoKontrol SENDING? I don't want to know what it's supposed to control in Mainstage. I want to know what is coming out of the NanoKontrol. Without this I can't help you any further.
There is a myriad of things that could go wrong here; one possible scenario is that the NanoKontrol is sending two different commands: one when pressing the button and one when releasing. All this can be configured somehow but I really need to know the current situation first. Or perhaps it actually is a bug in Mainstage but without knowing what kind of data is processed it's equally impossible to say.