In version 3.13, the Druidic Lore bonus seems to break once pottery is researched. It works fine until pottery is researched, and then reverts to the vanilla +1 faith if adjacent to a Forest/Jungle.
The code has not changed for this trait since version 3.7. Are you sure you are not just seeing the change from an unimproved tile to an improved tile?
In version 3.13, the Druidic Lore bonus seems to break once pottery is researched. It works fine until pottery is researched, and then reverts to the vanilla +1 faith if adjacent to a Forest/Jungle.