Problems with depth buffer only pass on iPhone 8/8+

In this test, we have eight teapots, each with different configurations, showing various issues. Originally, we were using Multi-Pass shaders (5-8 below), but as you'll see, on iPhone 8/8+, it's showing strange patches that render the background instead of the model. On other devices, these render correctly. We experimented with single-pass shaders (1-4 below), but when they are set to transparent, we see strange unexpected polygons.

1. Single-Pass, Culled, Solid There is no visible problem with this shader when not transparent.

2. Single-Pass, Culled, Transparent Part of this shader is not culling everything we expect. At certain angles, you'll see polygons that you shouldn't be able to see.

3. Single-Pass, Not Culled, Solid There is no visible problem with this shader when not transparent.

4. Single-Pass, Not Culled, Transparent This shader shows strange polygons when transparent.

5. Multi-Pass, Culled, Solid The object is rendered as desired, but there are strange patches that render the background instead of the object.

6. Multi-Pass, Culled, Transparent The object is rendered as desired, but there are strange patches that render the background instead of the object.

7. Multi-Pass, Not Culled, Solid The object is rendered as desired, but there are strange patches that render the background instead of the object.
   

8. Multi-Pass, Not Culled, Transparent The object is rendered as desired, but there are strange patches that render the background instead of the object.

Has anyone else had any shader issues on iPhone 8/8+? It seems like it might be related to this pass:

Pass { ColorMask 0 }

We're not sure why though.