Okay, so essentially fog cloud is bugged to never initiate combat?
Because it is an area that causes blindness, creatures that are blind automatically fail any check that requires sight, when you swing on a creature they make a perception check to spot you, so essentially it is working as intended, but it is very game-breaking.
Edit: Though if they had more intention to following 5e rules, in the errata they did fix fog cloud so that it only causes blindness to creatures trying to see through it, meaning they are only blind on their turn. If they used that rule instead it would fix a lot of the abuse around the spell.
