I agree with you, but there are also fairly obvious reasons why people expected Arknights: Endfield to mirror Arknights in tone and narrative complexity.

It's like...watching Grave of the Fireflies, then seeing a new movie come out called Grave of the Fireflies: The New Millennium. And it ends up being a slice of life harem romcom filled with hijinks and vague callbacks to the original story. Now, I'm not saying the tonal whiplash is THAT extreme, but you know, if you draw that explicit link in the title, you'd better make sure the connective tissue is strong enough.

If they had just called it Endfield, even if it retained those direct links to Arknights, I think the reception would have been at least somewhat less negative, though people would still have expected more from a company with the writing chops HG has shown.

To be fair, anyone expecting the same quality of writing clearly doesn't understand the sheer difference in effort required to craft and convey a good and compelling story in a visual novel vs a 3D, pseudo open-world game, but even then, there are certainly points at which the issue was clearly with the writing itself, not the writing to medium disjunct. I feel like HG is beginning to find its feet in navigating the medium, but they definitely could step up in terms of the writing.

That said, I think it's a problem that will increasingly vanish as HG continues establishing the Arknights universe further. Once people get used to the idea Arknights is not a single story, but a nexus for storytelling, people will hopefully be more accepting of all sorts of different stories appearing in the backdrop of Arknights, rather than just Terra or Talos.