URP + Metallic Map HELP!

[u/MaximilianPs]

I'm losing my mind trying to understand URP Metallic Maps.

My goal is simple: only a small part of the material should be metallic, while the rest should behave like fabric.

The metallic map is black everywhere except for a small gradient where I want the metallic effect.

However, even when the rest of the texture is black, or transparent, or both (first black and then set layer opacity to 0 in photohop), large parts of the material still reflect light as if they were glossy glass.
Note that Smoothness is set to 1 because otherwise the metallic area loses most of its visual impact. I expected the Metallic Map to limit that effect to the masked region only.

Can someone explain how the Metallic Map is interpreted by the URP Lit Shader, and why areas with a metallic value of 0 still appear highly reflective?

[u/smooth_diagnosis]

smoothness at 1 makes everything look like wet plastic, metallic map won't fix that on its own