Some of the other devices I tried are:
Motu Micro Lite 5x5
Motu Fastlane
Emu XMIDI 2x2
They all worked when the driver was installed correctly.
I won't discount the possibility that part of the error was on the side of the PC running Medialon. However, I uninstalled, wiped the registry, and reinstalled the PC drivers. When I could see consistent MIDI data from a random MIDI device, I was relatively satisfied that the PC was not the issue.
I think bottom line is that USB to MIDI through a wire then MIDI to USB is just nuts anyway. Things are working now and reliable across many different MIDI devices so at least my issue is closed.
I wouldn't be surprised if people have this same problem in the future.