The whole Shogun 2 launcher update some while ago has ruined the interactivity between the Steam Workshop and and the new launcher, meaning the previously functioning map mods on the workshop now no longer work. 

We can't use the map mods on the workshop anymore.

Still no news about fixing the mod launcher

I had the same idea, the manager needs to be updated.The Mod Manager only handles mods and not maps, which might be causing unable loading custom maps.

My suggestion is to add map management functionality to the Mod Manager. It would make it much easier for us players to use custom maps and fix this problem.

By the way, I wanted to mention that even though Total War: SHOGUN 2 was released in 2011, it's gaining popularity again thanks to the Shogun TV series. This is a great opportunity to update the game and cater to the new wave of players.

I hope devs consider my suggestion and address this issue.