Principled BSDF background not transparent

I’ve been trying to do what I thought would be a basic thing of creating a transparent Principled BSDF and seeing a transparent background, but strangely enough, it doesn’t seem so basic.

It’s a Principled BSDF with transmission at 1, refraction turned on for render and material settings, “Transparent” enabled under Film. The shader shows the object behind, but weirdly decides to show the environment map instead of a transparent background.

Sample file.

For context, the original thread on Blenderartists.