I feel like that's kinda the point- the melancholy comes from our journey with Frieren coming after the adventure is over.While I like seeing more backstory for Himmel and Heiter, I can't help but feel like the vibe is completely off in this side-chapter. I'm so used to the melancholy tone of the main story that this just feels very, very strange to read. Even thought it's Heiter telling Fern about Himmel, and it should be sad, I just couldn't pick up on it at all. It's, like... Upbeat? I guess. Defs weird, and I don't think I like it
In this flashback, we're still in the starting phase of the adventure, with the unknown and endless possibilities still ahead. It makes sense it's a lot more upbeat.