I think you are missing the issue I'm experiencing. Here is the patch:
LFO CV out (frame 1) into Filter CV in (frame 2)  via the front patch panel using standard grey cable (whatever length that is)

This was a working patch when everything was on  the same frame.  Frame 1 is the original frame where I made the patch.  I've simply moved the filter to Frame 2.  Now the patch will not work.  Filter works fine on the new frame without the CV source.

The problem I have is the signal is not making it there correctly.  The only change in output to the filter is a hum with the CV IN plug being put in or out.  When i move the LFO to frame 2, the patch works as expected. (no adjustment to settings).  I've tried adjusting the knobs with the LFO / filter in separate frames in case but that does nothing.

I'm certain it is not an issue with the new frame as I can put the modules completely on either frame and not have this issue.

I stated in the original post I am not using the bus for the signal and as you stated neither module would be using it anyways.  It leads me to believe A-185 is not the solution (unless I'm unclear on that module's function being bus only).

I've ruled out electromagnetic interference (rack hum) from each other as the modules work just fine in one frame or the other.

Attenuation of every cross frame patch doesn't seem the likely (or practical) solution.

As I've just outgrown frame 1 this is the first I've encounter something like this.