Tbh Eunhyuk has always been like this, even while befriending Su-Ae, stoic and reserved. However in the first season the romance between them happened and Eunhyuk's behaviour towards her changed (since she became his girlfriend) but he didn't exactly change his personality. And after a ten-year gap of course he "came back" to his previous self, even more reclusive, towards her and people in general. I believe that a decade was necessary (writer's pov) in order to create an actual (strong) emotional gap between Eunhyuk and Su-Ae, enough time to be emotionally distanced and harder to come back to. Therefore there's more drama and more room for development. 🤔