Hello, thank you for your reply. Yes, that seems to be the problem. There is a post about this subject here: https://community.vcvrack.com/t/making-your-monophonic-module-polyphonic/6926 And the output line looks like this:
outputs[SINE_OUTPUT].setVoltage(vcoEngine[c].getSine(), c);
So not only the output is multiplied by c, the voltage also depends on c. I will look into it further from here, thank you for pointing in the right direction.