Note: This is a know issue to DOS2DE and not exclusive to this or any other mod, but rather is an issue with the game-to-workshop interactions.

Many thanks to LaughingLeader for finding a solution and posting it here. Much of the section below is copied from his thread.

What is happening?
When you subscribe to a mod on the workshop, it's downloaded to the following folder:

Steam\steamapps\workshop\content\435150

When you launch DOS2DE, it copies mods in your workshop folder to the corresponding mods folder:

C:\Users\[Username]\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\Mods

Unfortunately this doesn't always happen for some people. Some possible ideas on why this may happen:

• Your Documents folder varies from the normal pathway.
  
• You have non-latin symbols in your username.
  
• You launched the game too soon after a mod was downloaded.
  
• There's some sort of permission issue when DOS2DE goes to copy a mod.

At the very least, knowing where mods are originally downloaded should allow you to copy them in manually, until Larian figures out a solution.

But the pak file is in my mods folder
Sometimes there seems to be something out of sync with DOS2DE and Steam. The mod (pak file) shows up in the correct folder, yet still does not show up in the mod list. In this case, try:

• Close DOS2DE
  
• Delete the mod (pak file) from the Mods folder
  
• In Steam, unsubscrbe from the mod
  
• Refresh the screen and re-subscribe
  
• Launch DOS2DE and it hopefully shows back up in the Mod list

I need my mod now!
Either:

• Find it in the workshop/content folder and copy to the correct Mods folder, per the above
  
• Go to NexusMods[www.nexusmods.com] and manually add the mod