Steam Workshop not so good for modding. It force to use own folder (also bad designed mod manager improved year ago but still). But that not the main issue.
What that affect is more folders needed. For game you need game folder in specific path
X/Games/SteamApps/common.

Steam client also add requirements for SteamApps/Workshop/ folder be on X/Games and not on C/ drive.
(actually whole Steam also shouldnt be on system drive).

As i understand game still able to refognise AddOn folder inside own root folder. By default it use Steamapps/Workshop.
In any way. If anything on rights restricted location - you can edit xml. Game still not see it. Instead Windows will copy and redirect to another file.

Go to user AppData Local VirtualStore and look if there is redirected file there. You need edit that one. In future better to reinstall everything. It is important Steam and SteamApps/Workshop also be not on c/ drive. For few reasons.

(one of steam forcely for no reasons can use own subfolder main SteamApps temp folders which also restricted if just only Steam in Progran Files, Steam randomly chose which SteamApps use to download files)(game exe can be set to admin run, steam.exe overpass it as all games actually steam.exe-rungameid run argument)