I found another strange bug I think. When you set the filter to lowpass with the cutoff very low, and also set the EG->VCF to zero, you hardly hear a sound. The filter is closed and the envelope doesn't open it.
Then play a very low note and the filter opens up like the EG->VCF value suddenly jumps to a maximum. Set EG->VCF to zero again and the filter closes again. 2 notes that seem to do this for example are A0 and C0, but maybe anything in octave 0 will do.