I simply do not create Thumbnails/WorkshopItems at runtime. In our case, which is the case for most people, we have a mod kit that is exposed to the user that is used to create mods.
If you are willing, I can help you with a TeamViewer session (send me PM).
Is your project C++? Set breakpoints in
SteamWorkshopManager.h - FSteamWorkshopManager::CreateWorkshopItem
SteamWorkshopAsyncObject.h - FSteamAsyncObjectCreateWorkshopItem::OnSteamCallback_OnlineThread