New freightliner cascadia air compressor continues running

Soapy water in a spray bottle.

 

I'd start by building up the air to cutout pressure. Then shut down the engine and listen around the engine/compressor area all the way down to the air tanks and dryer. See if you can hear any leaks. Grab a spray bottle with soapy water and spray down all the lines and fittings associated with the compressor/governor/dryer/wet tank.

It'd have to be a pretty sizeable leak to drain the wet tank 25 PSI in 1-2 minutes. Could even try and open the tank drain valves after its been sitting over night. If the wet tank is empty/sounds like it has less pressure than the primary/secondary tanks then you know for sure there's a leak.

 

The line to the governor that tells it to cut in is leaking, air dryer has no control over when it unloads.
When air governor gets signal and starts air into tank the leaking line get to pressure quickly and unloads the dryer.
FYI the air compressor never stops running, the governor just redirects the air into the intake.
Tell the dealer to bring in another truck and show you that it works just like yours.
Remember where you truck was assembled.