Paul, that's correct, the QMX is not powered by USB. Regardless, there will be 5V coming from the computer to the VCC pins on the USB-C pads on the board.Those VCC pads on the board should not be connected to anything, however if they were to short out to ground, there will be USB problems on the computer end.
So to answer Ian, with USB-A unplugged from the computer, you should not see voltage coming from the QMX. But it is at the QMX that the potential to short out computer USB power exists. I think before driving yourself nuts probing, you should try my previous suggestion and clean up the USBC pads, add flux and solder and reflow, getting everything nice and shiny (and clean! No solder bridging!). See if that gets things working. Also make sure you computer's USB port still works.
