Custom stencil broken for mobile in 4.21?

Has there been any changes in 4.21 that require extra steps in order to get custom stencil to work for mobile, or is this a bug? Porting a project from 4.20 that had a post-process material using custom stencil stopped working in 4.21. I also created a 4.21 blank project to test this and got same results: SceneTexture:CustomStencil outputs a color value of zero instead of the custom value assigned to the object. This happens both when tested in editor with Feature Level ES3_1 and also when running on an iOS device. Feature Level Shader Model 5 works fine.

Post process material used:

Feature Level SM5 (works fine):


Feature Level ES3_1 (doesn’t work):

Same setup as above but in 4.20 and Feature Level ES3_1 (works fine):

This has been identified as a bug/regresion and a ticket has been created by Epic’s staff: Unreal Engine Issues and Bug Tracker (UE-66502)