Wrong camera width/height with vuforia engine

I've strange (for me) behavior with vuforia. I use unity 2019.1.12f1 and vuforia 8.3.8.
When i turn off vuforia engine or turn off play background video in vuforia settings everything looks as expected as well as in unity camera preview.

But when play background video is turn on, devices screen looks as shown on second screen shoot.



It seems, that scene camera resolution changed to 480x240px, and streched out to screen resolution. I look camera and vuforia settings and didn't found any parameters for this.
What did i wrong?
Could anyone help me with my newbie question?