GFCI Outlet Hard Fail
Yeah, the Leviton rep didn't understand.  Perhaps you should change that to a deadfront if it's a dedicated circuit as you say.  

But congratulations, you have overcome the usual GFCI fallacy: "The GFCI must be defective! MY appliances don't have ground faults, those only happen to other people!"  

Indeed, I would follow SOP when chasing a GFCI trip: first remove everything from the LOAD side, see if that clears it.  If it does, divide and conquer the LOAD side appliances and wiring until it clears.  What you narrow it down to is faulty; you can either try to fix or replace it...

... or determine that GFCI is not required or useful for that device.  Which is what you do with refrigerators or freezers.