Question GrabPass shader intermittently interfering with world space UI

Using built-in rendering, Unity version 2019.4.40f1.

I'm having a dreaded intermittent issue. As you can see in the video, I've got a floating world space UI canvas just beyond the car's windshield, and am using a shader created from this tutorial

which does raindrops which displace pixels from GrabPass. This sometimes works fine, but other times the UI canvas just disappears where it intersects with the raindrop material. It's somewhat correlated with camera distance, I finally realized--by moving the VR headset nearer and farther from the windshield I can get the issue to occur or not, but still unpredictably.

Is there any kind of cycling function within the pipeline that might interfere with render order, or the functionality of world space UI, corresponding with camera distance that anyone knows of?

 

SOLVED! The answer was to set the Canvas / Render Mode / Order in Layer from 0 to -1.