I’ve solved the line flickering! I had a Post Process Volume that set the Bloom Method to Convolution. Setting that back to the default Standard method cleared up the lines. The flickering of that mystery plane is so intermittent that I have not yet had a chance to see if it returns with this change. It usually doesn’t happen unless I’ve been working in the scene for a while.
Question for you @alexandrecollaco - are you using Datasmith files in your scene? We suspect there are issues when using Raytracing in scenes using Datasmith imports.