@52beats thank you again for your help, and I hope you still have a bit of patience left with me. 
Can you try downloading the wav file I posted yesterday and testing it with Poly, please? It comes from My pack of 100 free Single Cycle Waveforms + Tutorial.
In your first picture with Poly, the length seems to be at 100% and the problem I’m trying to understand happens when you reduce the Length. The second picture isn’t Poly, and the case for single cycle waveform makes more sense for Poly, to get a simple and easy to play polysynth.