Dreaming about an ex is one of the most commonly searched dream topics anywhere in the world, and both Korean folk interpretation and general dream psychology land in roughly the same place on it: this dream is usually read as being about closure, not about the actual person or any real wish to get back together.
A calm, friendly interaction with an ex in the dream is read as closure genuinely achieved — whatever needed to settle about that relationship has settled, even if you haven't consciously thought about it in a while. An ex returning distressed, or trying to reconcile within the dream, is read as unresolved feelings still lingering somewhere under the surface.
Fighting or arguing with an ex is read as unprocessed resentment rather than current romantic interest — the dream logic here isn't "you miss them," it's "something about how things ended still bothers you." Seeing an ex who appears happy and has clearly moved on is read gently, often as your own subconscious giving you permission to do the same.
Interpreters on both the folklore and psychological side agree this dream says considerably more about your own emotional processing than about the ex-partner themselves, which is worth remembering before reading too much into who specifically showed up.