@Short
It's depend on the map you play. I'm sure that both pumpshotgun and chrome shotgun has alternate skin that resemble to one and another since 2020 update. So I think the map is think my custom skin is a chrome one. Yeah, that is Valve's fault anyways.

If you really want only one skin and not others. There is QC file inside the mod's vpk. Open the qc file, find and delete "$texturegroup" on both view and world's qc file. Recompile the qc file via Crowbar tool and repack the mod into vpk. (BUT DON'T POST ON WORKSHOP)

Here is guide for map modder about TLS gun skin:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2243203758

Ah that makes sense so it isn't RNG, but would it be possible to ensure the only gun the spawns is the "default" ?

@Short
These aren't RNG. I use this mod to make the custom skin random all at one, otherwise it always show default (black and white) skin unless map modder put a gun spawn with different skin on it.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2937384360

There are multiple sets? for the same skin? at random? could we get a separate mod for only the black and white ver? or what file should I keep when editing it on my end?