We'll use Let There Be Tooltips - Definitive Edition as an example.
If the mod does appear in the list, Steam did not install the mod correctly. To work around this bug in the Steam client:
%PROGRAMFILES(X86)%\Steam\steamapps\workshop\content\435150\1506230499\LetThereBeTooltips_c03acb8a-03b9-4c79-84ff-3784e97774da.pak
.%USERPROFILE%\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\Mods