This is not a bug but the outfit parameters of nearly all functions changed:
Most outfit functions need an actor parameter now as outfits can be used on the player and the companion!
Always make sure you check the "most up2date" API functions file linked at the Wiki for the current game version:
https://wiki.esoui.com/APIVersion#live_API_version
-> The link at "API TXT Documentation"

Open the txt file and search for the function, and check the parameters and values needed
Lua Code:
* GetOutfitName(*[GameplayActorCategory|#GameplayActorCategory]* _actorCategory_, *luaindex* _outfitIndex_)
** _Returns:_ *string* _name_
Make sure to check all outfit API functions and usages of ZO_OUTFIT_MANAGER (like function EquipOutfit) etc. for the first parameter GameplayActorCategory now. Possible values are:
Code:
h5. GameplayActorCategory
* GAMEPLAY_ACTOR_CATEGORY_COMPANION
* GAMEPLAY_ACTOR_CATEGORY_PLAYER