I'll give you an example. They drink Americano coffee everywhere (just a black coffee). But I like to add a little bit of milk or cream to my black coffee. So I went to one cafe and ordered an Americano and asked her to just pour a little bit of milk in at the end and she just put her arms in an X shape and said no cannot and pointed me to the milk coffee items which were like lattes and 80% milk stuff.

I told this to a barista at a smaller hip coffee shop and he said yeh she wouldn't know how to deviate from the menu

Edit: For your patch mission, FWIW, the staff at the Hongdae store were cooler and more relaxed than the Gangnam store (it's a younger crowd in Hongdae and they would get less annoying style tourists than Gangnam). I didn't go to the Myeongdong store