Denial

I have been playing with the Synthesound cartridge and found a peculiarity with it and I am not sure if it is the normal behaviour or there is a fault with the cartridge.
When I first start, if I disable all but one voice I can play one voice. But if I start with more than one voice I cannot turn the extra voices off from this point in programmer mode. If in fact I turn all the voices off in this mode I just get a continuous note back in player mode.
I discovered however I can turn all the voices off in sequencer mode.
Anyone else used this cartridge and have any advice about it?