Only things I can think of are its either a defective adapter, defective UMC, or you are mistaken about what a 14-50 outlet/plug look like and you really have a 14-30 or something like that. It wouldn't be the first time that mistake was made.
I guess there's a very tiny chance its a defective charger in the car, but they fail in 16 amp increments usually. I guess maybe that could have changed if you have the new battery pack chemistry.