Surface Shader Layer only visible in Sprite Mask view

I started having this weird shader issue about a week ago. I made a custom Surface Shader to test clear coat rendering, something I've done before with success. I did it by stacking two surface programs within one shader script, but for some reason the second layer isn't displayed. Neither in Scene View nor Game View/ gameplay. But it is visible in the Inspector and, strangely, the Scene View Sprite Mask Draw Mode. I have no idea why it's visible in Sprite Mask view but nothing else. And as of yet I haven't found any info on the Scene View's Sprite Mask draw mode, not even in the Scene View control bar manual. There are no shader errors present, and I even made another identical but simpler shader with the same result.

It first appeared in Unity 2017, but since then I've updated to 2018.3 and it persists. As far as I can remember, I haven't made any strange changes to Unity's render or lighting settings, but is there any setting that affects shaders like this? Any help is appreciated.

Visual example, material with shader shown in Game and Sprite Mask views, and Inspector: