Studio's motion tracking leaves a lot to be desired. I spent a considerable amount of time adjusting trackers to focus on the license plate. It was blurred the way I wanted but then the mask turned black and is unstable. See link below.

The latest version of a competing editor has a vastly superior motion tracker if anybody needs the feature.

 I have it working on a clip from the sample movie but again it required lots of tracker tweaking.  

https://youtu.be/IzUTIRTKRB0