Yes, this is a giant light block. In the first picture I am holding a light block and a giant one comes up and fills my entire screen. Below is a build I have finished and I am lighting it up with light blocks. Down there are some real light blocks, for size comparison. I started to notice these yellow flickers while I was building and tried to see where it was coming from. In the second picture, I am holding oak wood planks (there is no render when I am not holding a light block) and the render goes away.

Note that I used about a hundred of them in my build. Don’t blame me, it is a big build.