Let me explain it to you

Why this is happening

1. The material has specular enabled, which means it will be reflective and look metallic or shiny

2. Specular reflection is based on map HDR and becomes strange if the map doesn't have HDR or misses the material

3.$envmap That's how he works

4.mat_specular 0 turns off specular reflection

So you can try to change the map, maybe it'll be good, the initial map is too old, it's classic, but there are shortcomings

How it works [$envmap] [HDR]
https://developer.valvesoftware.com/wiki/$envmap

You can access the secret room of the initial map, where there is a separate copy of the HDR

Thx!