"There is an issue for some people where the game stops synchronizing mods recently.
I believe this is caused by one of the mods being removed from the Steam Workshop but not automatically unsubscribed, and the game was not prepared to handle this case.

Here’s how you can fix it:

• Open Steam and go to your game library.
  
• Right-click on the Dwarf Fortress game.
  You should see a small pop-up window with information about the game installation (this is the same place where you can select the beta branch).
  
• Go to the Workshop tab.
  In my case, I found one item that had been removed from Steam, leaving an empty entry.
  
• Click the trash icon next to the empty entry to remove it.

I’ve attached a screenshot for reference. Note that it’s in Polish, and the removed mod was "mocked" in Paint since I took the screenshot later.
If you have the English version of Steam with the removed mod still subscribed, please attach or link it for a more helpful reference."