I’m not sure if what I am thinking it what you mean but maybe it is :)
for both of these the ost definitely had a huge role is making it feel very nostalgic to me but also the cinematography and particularly in The Forbidden Flower the colouring over some scenes in the beginning made me feel like I was in a dream remembering beautiful memories. When I say nostalgia I don’t mean that I relate to anything that is happening in these shows but just the way they were put together gave me that feeling that you get when you are nostalgic.
I’ll See You In My 19th Life - Kdrama
The Forbidden Flower - Cdrama
both of these are romances