It's hard to imagine Deun Yu voluntarily leaving Wong Yu Yin's side, but that's exactly what he did on that one occassion after Pau But Tung wouldn't stop insulting him. Despite his infatuation with Wong Yu Yin, Deun Yu couldn't stomach being constantly insulted by Pau and just left.
Deun Yu was in Goo Soo at the time, and after parting ways with Wong Yu Yin, Pau But Tung, Ah Chu, and Ah Bik, he wandered farther north to Gong Nam, where he met Kiu Fung.
Why didn't Deun Yu head home to Dali instead of wandering so far north? This was the second occassion in which he had left home, but unlike the previous occassion, his departure from home the second time was involuntary. He had been kidnapped by Kau Mor Tze at the Celestial Dragon Temple and taken to Goo Soo; he had not left Dali of his own volition.
If he wasn't going to follow Wong Yu Yin around, then logically, he would have headed home to Dali. Why did he aimlessly wander until he got as far north as Gong Nam?