It's mostly either details, like the scene where Reynold lightly pinches Penelope's cheek before the coming-of-age ceremony to tease her for example. Also, I wish we could have seen the knights' faces (maybe we'll see them in the side story) and Callisto's mother too (idc about Penelope's OG family since we'll see them again anyways and Callisto's father was absent until his death in the novel too)

It's personal but some scenes gave a different feeling in the manhwa compared to the novel?

Like the scene just after Penelope collapses from the poisoning and Callisto tries to keep her awake (it felt more tender and bittersweet/full of longing in the novel while in the manhwa, it was more dramatic in the sense that it you could feel the tension and the stakes were high).
It's not necessarily bad, it's just different! And I feel like the scene where Callisto's "don't leave my alone in this hell" had more impact in the novel while in the manhwa, the season finale scene felt more emotional and impactful (probably because it was the penultimate scene of S04).

I think the details/scenes you specifically mentioned had to do with the fact it happened around S05 Part 1 finale/S05 Part 2 start (I don't think it was the artist's decision to divide into two parts) so the scene lost of its fluidity and instead went for a dramatic impact.

It's still a pretty faithful adaptation (I know not everything can be the same), I appreciate some of the changes: Penelope under that altered appearance on the boat instead of just wearing a cloak, more insistance about Eorka's founding legend/the Golden Dragon Tooth, more Winter's lore (directly linked to the timeloop plot), the scene where we get the clear signification of the rose colors... I think it's necessary because some things that are (heavily or not) implied in the novel have to be shown in the manhwa so the readers don't get confused. Also, Penelope is slightly softer in the manhwa but I've already said I do get why and I don't mind!