The fact that this is the .5 chapter seems nearly disrespectful. What a beautiful story about grieving and family. I wonder if it could reach more people if it was a standalone oneshot. Really hope more people find this gem in the future.
It was probably a one-shot that was published elsewhere (Office YOU, or other magazine). Normally one-shots like these are compiled in an author's series volumes to fill space and probably having a chance to be published elsewhere in their catalogs. Unfortunately, not every one-shot makes it to be in a single anthology by their authors (and the less popular or active, the worst chance for it to happen), or else, and stay in its original format for a longer time (unavailable) after its original publishing date.
This was (re?)published as such in the original series' volumes. Would have been okay if this was by separate? Yes. But the same could be said for the many other single one-shots that were fantranslated years and years after the fact, and the only material people have to work with is from a series' volumes themselves.
I don't think this is "disrespectful", although I understand the sentiment of this being packed in this one series, for how it touches grief and mourning. It also kind of fits with the whole subject of the series "And it will be sunny".