Sorry, you are having issues with your instrument.
Question: are you connected to a computer or iOS device at the time?
If plugged into Foot Controller 1 (Expression) is the pedal erratic as well?

Shut the computer off. No joy. Moved the pedal to FC1 and assigned the superknob to it. No luck there either. Trying to figure out if this is more likely to be an FC7 issue than or a MODX problem. The FC7 is right out of the box....

Assuming you have nothing plugged into MODX except a power cable and the FC7 and also assuming you are testing the foot controller with factory preset Performances - there are really only 3 options of what's going on:

Both of your MODX ports are broken (not very likely),
Your FC7 is broken (bad cable or dirty potentiometer),
or your FC7 isn't plugged in correctly (fully) - which is also unlikely given you've moved the FC7 to a different port - unlikely you'd make that "mistake" twice. Also, in general, unlikely anyone would have this problem given how similar an FC7 connection is like so many other electronic devices any consumer has encountered in their past (and therefore trained).

If I was trying to settle this - I'd take my FC7 that's part of a misbehaving configuration and carry that same FC7 into a local music shop that has a MODX. I'd see if the problem follows the FC7 and acts up in the shop. If it does - I may inquire if they have an FC7 (the shop's - new boxes open and close without issue) I can try on the floor model and see if it works better than my own misbehaving FC7. If results change (are fine with the shop's) - then I'd conclude I need to return/exchange the FC7 I brought into the shop.

Not all shops have all accessories - so YMMV with part of the above testing. But that's how I'd go about this one. Always checking factory presets - not to introduce any extraneous variables. And initializing the shop keyboard just to make sure there aren't any rogue settings.