Depth Rendering Issue

Hi! I have possibly a fairly specific question but would love any help I can get. I'm using Unity to visualize a volume from a set of 2D images with the help of this open-source volumetric library: https://github.com/brianasu/unity-ray-marching/issues/

I am however having difficulties with depth buffering. If I try to put an object such as a primitive cube in front of the rendered volume, the volume is always rendered in front. I have recorded a brief gif of the issue occurring here: https://gfycat.com/NastyHeftyItaliangreyhound

Unfortunately I don't know enough about shaders or the graphics pipeline to understand exactly what is causing the issue and how to fix it.