Red Blossom has some good info, but it's a leap to assume that because Taiwan lacks a "milk culture" that the milk flavouring in flavoured Taiwanese Jin Xuan is as artificial as it gets or uses diacetyl. (Taiwan does use dairy in various foods and beverages these days, including with tea products).

Not all flavoured milk oolong has that trademark buttered popcorn aroma and taste either. I've encountered some that clearly taste and smell of steamed cow's milk or even fall into tangy cream cheese territority (similar to "cheese cream" foam you'd see at bubble tea places). A lot of countries (Vietnam, Thailand, China) grow the jin xuan cultivar too and there isn't a homogenous way to flavour them. What-Chacarries a flavoured Chinese Jin Xuan that tastes a bit like mangos and milk. Other unique twists to flavouring Jin Xuan include rose, jasmine, and sticky rice. It's an art in of itself.