Excellent problem-solving. Thanks for sharing the solution you found. I have used loopMIDI, but not in the way you do. I have also used MIDI-OX, but mostly as a diagnostic tool.
I think (not 100% sure) the reason I mainly use MIDI-OX for diagnostics is that when I use it, some internal routings cannot happen. That might even be what your issue was. I never tried to debug it, though.