I used Vroom, not Carvana, but had the same problem.
While part of it was Vroom’s fault, I will say that about 50% of the fault lies with the State of Missouri.
I am sure that neither company is willing to throw the Department of Revenue of several states under the bus, so they are taking the hit full on.
I think since Carmax sells the vehicles out of a brick and mortar in the state that they are registering there isn’t the same problem.
Carvana and Vroom are 100% online and delivery and can come from numerous states. My car came from California but the original Temp Tags were from Texas. This could also be “growing pains” in figuring out the intricacies of different state licensing systems.