Heesun and Subin Broke Up
As per Heesun's latest video she mentions that she wants to find a good guy in the future, also mentions she's been through some hard times and that was a clear subtle hint at their break up.
For some reason, I always thought Subin was not into her while watching the show, But, when the after episodes were released and I saw them being all lovey dovey and it was so clear that they are dating, I started to love them together! (They were spotted also few times going for park dates). It's such a shame things didn't work out.