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 https://imgur.com/NlHdIJo.png
effect camera https://imgur.com/ceqj7N5.png

normal screenshot https://imgur.com/Wfpbc5z.png
weird screenshot https://imgur.com/O9VTS2S.png 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
https://imgur.com/1GBmPMJ.png