Some workshop content is not a campaign but a location. Check under workshop the locations tab and you might see what you subscribed there. These are old efforts before TA introduced the custom campaign system. If you want to play multiplayer with that location, is suggested to create a campaign, import the location and then save it.

Ah, ok, so even though it appears as a mod, it could be a location in another campaign, if I'm understanding you correctly, which would make sense I guess. Thanks for the clarification.

No, its just that, a location. Its not one or one of many locations in a campaign. You can see the distinction with "Tomb of heroes". If you check the workshop item, it says on the right panel, it has tag "Userlocation". Whereas, "Temple of Elemental Evil" has the tag "Usercampaign".