Doesn't every truck pulling a trailer or 5er have the rear plate 'obstructed'?
You might get stopped because your trailer doesn't have a plate but not because the LEO can't see the truck plate. From behind there's no way of telling if you're from a state that does not issue light trailer plates, if you had a plate and lost it, or if you are running without a plate. You may get stopped to see why you don't have a plate.
WI is not the only state that does not issue plates for certain trailers. Most LEOs who work traffic know what states those are