Can you explain this Poly/Mono behavior?

[Perfect Human Interface]

I did up a little doodle here of the basic premise rather than uploading my whole project.

http://s21.postimg.org/47etjv8dz/Poly_Mono_Schem.png

PolyMono Schem2.png (23.24 KiB) Viewed 5516 times

As you can see there's a poly stream that's split across 3 oscillators, then each stream is converted to mono and the signals are combined at the end. Oscillator 3 has switches (selectors) on both the input and output which switch the stream off.

Here's the thing: If switch A is on AND switch B is off, no poly data goes through to any Osc whatsoever (expected behavior is for the poly data to go through Osc's 1 and 2 and output audio). If both are on, both are off, or if switch B is on but switch A is off, poly goes through and I get audio output.

Can anyone explain why? For some reason poly2mono has to connect up with other streams from poly2mono's or poly stops working?

[Perfect Human Interface]

Just want to point out that I circumvented the issue by using a selector to multiply by 1 or 0 in the second switch rather than "selecting" the stream off.