WebGL artifacts on Unistorm demo scene

1. What happened
While playing one of demo scenes from UniStorm A$ package in WebGL, terrain blinks white when looking around. This appears to be a regression from 5.4.3p1. At least for default player settings - didn't try changing anything there.

2. How we can reproduce it using the example you attached
- Build WebGL player for 'Advanced UniStorm UI Control C#' scene (or see Web1 folder inside the project folder)
- Play
- Walk around and glance up/down/far-right/far-left
---> Terrain turns white for fractions of a second, but it happens quite often.
Video: https://oc.unity3d.com/index.php/s/N1Rm68dTXO8eXqw

The same scene looks OK in 5.4.3p1