As of recently, it looks like both MarketplaceService:GetProductInfo().IsLimited and MarketplaceService:GetProductInfo().IsLimitedUnique are incorrectly returning false values for limited and limited unique items. Nevertheless, there are valid values being displayed for CollectibleItemId and CollectibleProductId and ProductType, which indicate that the item is indeed some sort of limited item.
Here are 2 examples:
Dominus Empyreus (a Limited Unique item) shows false for IsLimitedUnique:
Golden Crown (a Limited item) shows false for IsLimited:
