Hmmm...I don't think this is filler at all, let alone romantic or lovey dovey. I know I'm not the only one who notices the uncomfortably strange undertone in this season so far. And it's actually all about character development and seeing both characters in a different setting under different circumstances. We're learning a great deal about Sangwoo that we didn't know in previous seasons. Seeing Bum apologizing repeatedly and Sangwoo imagining his mother, I feel on edge like something horrible is making it's way in soon.
But there is no development this chapter and most chapters of season 3 could've been half as long as they were and we wouldn't have missed anything in terms of development. I'm not saying that most chapters of season 3 were fillers but most chapters had filler material in it. Only chapter 42 was a complete filler because there was nothing new in it.
Hmm, maybe this isn't the last season or season 3 will be significantly longer than the last two and that's why there is so much filler material. And yes, episode 42 is a filler. We already know about Sangwoo's hallucinations and how they scare him and the lovey-dovey stuff was shown in great detail over the last few chapters. There was nothing new in chapter 42 an that makes it a filler.
Koogi probably agreed to make a certain amount of episodes, maybe more than planned because of how successful KS was. But because she already decided on how the story will end and she can't place the fillers anywhere else we now have many romantic chapters with little to no development.