Multiple Cameras Problem

Hello I have a problem. I have a main camera, and an effect camera, both connect to steamVR headset.
In the beginning, main camera is enable and effect camera is disable.
When I play, and enable effect camera, then happen a weird flash.
see video:
https://youtu.be/2jOZZwPcRQ0

here is my camera settings:
main camera Imgur: The magic of the Internet
effect camera Imgur: The magic of the Internet

normal screenshot Imgur: The magic of the Internet
weird screenshot Imgur: The magic of the Internet in time 0:04.
weird screenshot is temporary and continued 2~3 frame. But I still confuse why my game happens this problem

thanks all.

by the way, My Unity version is 2017.2.0f3
and other VR SDK version is