Custom Stencil Not Working for Mobile

Setting a custom stencil on meshes is not working when using a mobile renderer. This is working as expected in 4.20 CL# 4369336

Found in 4.21 CL# 4541578 and 4.22 CL# 4556068

This is a regression

1. Open the editor
2. Go to project settings and enable Custom Depth-Stencil Pass to "Enabled with Stencil"
3. Place a basic shape in the level
4. Enable "Render Custom-Depth Pass" and set the stencil to 1
5. Create a post process material to output the custom stencil scene texture

(Note that using the buffer visualizer for custom stencil won't work, as the texture is stored differently in the mobile renderer, and it won't show up correctly here)

Results:
The shape will not show up under the custom stencil pass when using ES3.1

Expected:
The shape would show up 

[UE-77463] Stencil not visible in ES3.1

Custom stencil broken for mobile in 4.21?